Mozilla has upped the stakes in its squabble with Microsoft over user choice, having released a new version of Firefox that can sneakily subvert internet searches fed to Windows 10's Cortana personal assistant.

Firefox 40, which shipped on Tuesday, comes with "a fresh new look for Windows 10" that includes "bigger, bolder design elements as well as more space for viewing the Web" – but that's only half the story.

It also ships with a feature that redirects Cortana's web search requests to the search engine you have configured as your Firefox default.

Windows 10 does let you choose which browser you want to launch when you do web searches using Cortana. But the search requests themselves will always point to Bing, even when you've chosen a different default in your browser.

Beginning with version 40, as long as you have Firefox configured as your default browser, it will grab those requests and reroute them to whichever search engine you have chosen in your Firefox options.

The move is just the latest stone Mozilla has hurled at Microsoft since the launch of Windows 10 last month.

In July, Chris Beard, the open source outfit's CEO, published an open letter excoriating Microsoft for switching unsuspecting customers' default browsers to Microsoft Edge when they chose "Use Express Settings" during the Windows 10 upgrade process.

"It now takes more than twice the number of mouse clicks, scrolling through content and some technical sophistication for people to reassert the choices they had previously made in earlier versions of Windows," he railed.

Using similar language to that post, Tuesday's blog post announcing Firefox 40 says the latest change aims to "preserve your choice when using the search field on the Windows 10 taskbar to search the Web."

Countdown to signed add-ons

Firefox 40 also brings the browser a step closer towards locking down third-party add-ons. Mozilla announced in May that it had begun digitally signing browser add-ons that it had vetted as being safe to use. Beginning with this latest update, Firefox will now display warnings next to add-ons that haven't been certified by Mozilla in this way.

The open sourcers have said they don't want to adopt the approach used by Chrome, where only those extensions that have vetted and served through Google's own distribution channel. But Tuesday's blog post added that Firefox will eventually disable uncertified add-ons by default – although that won't happen until an unknown, future version. ®