Update: I did a bit more filtering and checking and the numbers dropped to around 76%. Head down past the caveats for more.

This question gets asked quite a lot around Mozilla now that we’ve been working on WebExtensions for a while. We’ve been aiming for some sort of Chrome parity. A while back I wrote a script that I used on arewewebextensionsyet.com. That produced a percentage of WebExtensions, but I always felt nervous about talking about it because there so many caveats it never felt accurate.

One of the major caveats was that the sample size of extensions I had from the Chrome store of 10,000 felt too small (that sample is the one used on arewewebextensionsyet.com currently). This week I used this project to parse the sitemap on the Chrome store and get 100,000 extensions and apps. A much more satisfying amount.

After stripping out apps, I then ran some scripts over those extensions to see how many would work in Firefox Nightly. Here’s the output:

Number Percentage Extensions 71,551 Missing one or more permission 5,463 7.64% Missing one or more API 6,542 9.14% Easily convertable 62,792 87.76%

And here is an explanation of the long list of caveats and why I still am very cautious about this number.

Caveats

This doesn't mean they can run out of the box, it means they are </b>easily convertable</b>. Possibly the most obvious reason is that some of these extensions contain deprecated methods. Firefox hasn't implemented those deprecated methods. But if the developer updates their extension to use the new methods it will work in both Firefox and Chrome.

This method filters out apps, or extensions which use apps API, which can be a little heavy handed and hard for extension users to spot.

This method just looks at APIs and permissions. Which means that features that are required beyond those two methods are not covered. Web APIs for example aren't tested.

Whilst we've got many APIs, some of them may not be implemented exactly the same as Chrome. Or have bugs. That can cause some extensions to have some problems.

A grep of chrome.* for APIs reveals domains, typos, comments and some just wierd strings. If you ignore those there would probably be even more that work.

This uses data from Firefox Nightly, if you use earlier versions of Firefox, you'll get less. Also not every single API is ready for release yet.

The result of these caveats is a list of variables which you can see in the code. A more perfect scenario would be to download the extension, try loading it into Firefox and then seeing the result. But automating and building it all out would take more than one hour or two of hacking. If anyone wants to try to do that for 100,000 extensions let me know.

So what can you take from the number 87.76%? Some comfort that its probably somewhere around that number. I can say with a lot of confidence that over 75% of extensions are easily convertable to Firefox. A bit less confidence that over 85% are.

That sounds pretty good to me though and a credit to the hard work the team has put in this year.

Update: November 22nd

A couple of more issues or caveats were pointed out to me:

This list of add-ons includes themes, those are possible in Firefox already and although its related slightly different. Because themes don’t use APIs and are different they should probably be excluded. By the way, there is a plan to get new theming support in Firefox. This dropped our total number of extensions examined down.

This does not include manifest keys. Some elements of the manifest do not need an API or permission so if we don’t support them we still aren’t at parity. All the big ones, we already have bugs and plans for, but just to be complete I included these.

The result is now:

Number Percentage Extensions 57,804 Missing one or more permission 5,242 9.07% Missing one or more API 6,309 10.91% Missing one or more manifest 7,799 13.49% Easily convertable 44,716 76.36%

Told you I felt pretty good about 75% of extensions. For a more detailed output, check out the repository.

Full output of the script from today:

Importer stats -------------- 71551 74.81% success 24041 25.13% apps 56 0.06% error 5463 7.64% missing_permissions 6542 9.14% missing_apis 62792 87.76% easy_conversion Missing APIs ------------ 1369 chrome.desktopCapture.chooseDesktopMedia 533 chrome.permissions.request 458 chrome.declarativeContent.onPageChanged 454 chrome.declarativeContent.PageStateMatcher 443 chrome.declarativeContent.ShowPageAction 427 chrome.devtools.panels 360 chrome.identity.getAuthToken 346 chrome.proxy.settings 311 chrome.permissions.remove 292 chrome.devtools.inspectedWindow 291 chrome.desktopCapture.cancelChooseDesktopMedia 272 chrome.permissions.contains 221 chrome.management.setEnabled 187 chrome.runtime.error 185 chrome.identity.launchWebAuthFlow 182 chrome.management.onInstalled 168 chrome.tts.speak 167 chrome.identity.removeCachedAuthToken 146 chrome.browsingData.remove 141 chrome.tabs.cr_tabs_TabClosed_listeners_added 141 chrome.tabs.cr_tabs_TabCreated_listeners_added 141 chrome.tabs.cr_tabs_TabSelectionChanged_listeners_added 141 chrome.tabs.hasOwnProperty 128 chrome.identity.getProfileUserInfo 126 chrome.management.onUninstalled 126 chrome.devtools.network 116 chrome.management.onEnabled 109 chrome.manifest.content_scripts 107 chrome.extension.onMessageExternal 99 chrome.management.uninstall 94 chrome.downloads.onDeterminingFilename 94 chrome.management.onDisabled 92 chrome.extension.onClicked 78 chrome.experimental.cookies 76 chrome.proxy.onProxyError 76 chrome.identity.getRedirectURL 71 chrome.storage.managed 71 chrome.tts.stop 61 chrome.privacy.services 59 chrome.manifest.version 57 chrome.privacy.network 56 chrome.tabCapture.capture 54 chrome.pageCapture.saveAsMHTML 51 chrome.permissions.onAdded 50 chrome.tts.getVoices 46 chrome.browsingData.removeCache 43 chrome.d.ts 43 chrome.debugger.attach 43 chrome.storage.set 42 chrome.debugger.sendCommand 40 chrome.permissions.onRemoved 38 chrome.extension.connectNative 38 chrome.permissions.getAll 37 chrome.debugger.onEvent 37 chrome.debugger.detach 34 chrome.privacy.websites 33 chrome.i18n._getL10nData 33 chrome.i18n._setL10nData 31 chrome.identity.onSignInChanged 31 chrome.webRequest.MAX_HANDLER_BEHAVIOR_CHANGED_CALLS_PER_10_MINUTES 30 chrome.tts.isSpeaking 26 chrome.fontSettings.getFontList 25 chrome.storage.get 24 chrome.contentSettings.javascript 24 chrome.debugger.onDetach 22 chrome.webstore.install 21 chrome.cast.media 21 chrome.input.ime 21 chrome.contentSettings.popups 20 chrome.system.cpu 20 chrome.cast.SessionRequest 20 chrome.runtime.onUninstalled 20 chrome.declarativeWebRequest.onRequest 20 chrome.cast.ApiConfig 19 chrome.cast.requestSession 19 chrome.browsingData.removeHistory 19 chrome.system.display 18 chrome.contentSettings.cookies 18 chrome.cast.initialize 18 chrome.browsingData.removeDownloads 18 chrome.browsingData.removeCookies 17 chrome.tabs.TAB_ID_NONE 17 chrome.declarativeWebRequest.RequestMatcher 17 chrome.webRequest.ResourceType 16 chrome.browserAction.show 16 chrome.settings.uuuid 16 chrome.extension.getVersion 15 chrome.system.storage 15 chrome.tabs.executeScriptAsync 15 chrome.contextMenus.ACTION_MENU_TOP_LEVEL_LIMIT 15 chrome.privacy.IPHandlingPolicy 15 chrome.runtime.lasterror 15 chrome.tabs.executeScriptInFrame 14 chrome.example.echo 14 chrome.settings.get 14 chrome.runtime.Port 14 chrome.contentSettings.images 14 chrome.extension.onConnectExternal 13 chrome.ttsEngine.onStop 13 chrome.settings.set Missing permissions ------------------- 1382 desktopCapture 620 clipboardRead 616 declarativeContent 510 proxy 476 geolocation 457 browsingData 348 contentSettings 275 tts 232 system.cpu 158 privacy 156 debugger 151 pageCapture 144 identity.email 120 tabCapture 91 sessions 79 downloads.shelf 54 fontSettings 52 unlimited_storage 39 ttsEngine 36 system.storage 32 input 30 fileBrowserHandler 29 system.display 26 declarativeWebRequest 18 system.memory 14 processes 11 power 8 audioCapture 7 7 {u'socket': [u'tcp-connect', u'tcp-listen']} 7 videoCapture 7 printerProvider 6 location 5 webview 5 fileSystem 5 enterprise.platformKeys 4 create 4 experimental 4 infobars 4 {u'fileSystem': [u'write']} 4 signedInDevices 4 wallpaper 3 networkingPrivate 3 dial 3 cast.streaming 3 app 3 mdns 3 cast 3 streamsPrivate 3 keybinding 2 bookmarkManagerPrivate 2 certificateProvider 2 preferencesPrivate 2 activetabs 2 enterprise.deviceAttributes 2 {u'fileSystem': [u'write', u'retainEntries', u'directory']} 2 usb 2 * 2 dns 2 tab 2 audio 1 *//yousonnik.ru/* 1 all_frames 1 webR/equestBlocking 1 offroad 1 hid 1 @ant-sites-string-list@ 1 newtab 1 accessibilityFeatures.read 1 activeTab1 1 t 1 .entireweb.com 1 accessibilityPrivate 1 externally_connectable 1 brailleDisplayPrivate 1 alwaysOnTopWindows 1 Storage 1 mangement 1 clipboardWrite 1 https:ajax.googleapis.com/ 1 *.facebook.com/* 1 dataReductionProxy 1 desktopCapturePrivate 1 enterprise.platformKeysPrivate 1 popup 1 copresence 1 accessibilityFeatures.modify 1 platformKeys 1 beamrise 1 Tabs 1 http//gosnapshot.com/snapshot 1 s 1 urls 1 https: //*/* 1 commandLinePrivate 1 system 1 sienium 1 virtualKeyboardPrivate 1 www.bgu.ac.il 1 none