Firefox 44 has been released, and it has started warning users of Tab Groups about its removal in Firefox 45. There were a number of reasons that led to the removal of Tab Groups. This post will aim to talk about each in a little bit more detail.

The removal happened in the context of “Great or Dead”, where we examine parts of Firefox, look at their cost/benefit balance, and sometimes decide to put resources into improving them, and sometimes decide to recognize that they don’t warrant that and remove that part of the browser.

For Tab Groups, here are some of the things we considered:

It had a lot of bugs. A number of serious issues relating to performance, a lot of its tests failed intermittently, group and window closing was buggy, as well as a huge pile of smaller issues: you couldn’t move tabs represented as large squares to groups represented as small squares; sometimes you could get stuck in it, or groups would randomly move ; and the list goes on – the quality simply wasn’t what it should be, considering we ship it to millions of users, which was part of the reason why it was hidden away as much as it was.

The Firefox team does not believe that the current UI is the best way to manage large numbers of tabs. Some of the user experience and design folks on our team have ideas in this area, and we may revisit “managing large numbers of tabs” at some point in the future. We do know that we wouldn’t choose to re-implement the same UI again. It wouldn’t make sense to heavily invest in a feature that we should be replacing with something else.

It was interfering with other important projects, like electrolysis (multi-process Firefox). When using separate processes for separate tabs, we need to make certain behaviours that used to be synchronous deal with being asynchronous. The way that Tab Groups’ UI was interwoven with the tabbed browser code, and the way the UI effectively hid all the tabs and showed thumbnails for all of them instead, made this harder for things like tab switching and tab closing.

It had a number of serious code architecture problems. Some of the animation and library choices caused intermittent issues for users as linked to earlier. All of the groups were stored with absolute pixel positions, creating issues if you change your window size, use a different screen or resolution, etc. When we added a warning banner to the bottom of the UI telling users we were going to remove it, that interfered with displaying search results. The code is very fragile.

It was a large feature. By removing tab groups we removed more than 24,000 lines of code from Firefox.

With all these issues in mind, we had to decide if it was better to invest in making it a great feature in Firefox, or remove the code and focus on other improvements to Firefox. When we investigated usage data, we found that only a extremely small fraction of Firefox users were making use of Tab Groups. Around 0.01%. Such low usage couldn’t justify the massive amount of work it would take to improve Tab Groups to an acceptable quality level, and so we chose to remove it.

If you use Tab Groups, don’t worry: we will preserve your data for you, and there are add-ons available that can make the transition completely painless.