Guest mode is a feature to allow others to use your Firefox account without affecting your profile (e.g. history, bookmarks). However, I think it adds complexity to the code and test paths without much value. Notes: - It's used by a minuscule percentage of users. Here's a query with the absolute number of guest mode users: [1] - Our guest mode does not prevent a user from accessing other applications (or the non-guest profile) on the device, which are not in guest mode - Android (from some API level) natively supports guest profiles that are more comprehensive than our solution (see above) on the devices you'd want to share (tablets) - Private browsing is roughly the same as guest browsing, except that it doesn't hide some features (e.g. sharing, bookmarking) In theory, every feature that we add needs to be tested in guest mode, doubling the number of combinations to test. In practice, we don't do this and some bugs slip through the cracks. For example, I decided to file this when bug 1408846 uncovered that we did not set up the AS context menu to support guest mode in the way old top sites does. fwiw, there are 26 open bugs for guest mode at the moment [2]. [1]: https://sql.telemetry.mozilla.org/queries/4539#table [2]: https://mzl.la/2gKVu7P

+1 from me, at least from an engineering perspective. Guest Mode pushes tendrils everywhere -- we have to disable bookmark import from the OS, manage lock screen widgets, hide some UI elements (e.g., Synced Tabs) disable extension installs, block some URLs (e.g., about: pages), make sure we don't accidentally sync your guest mode tabs (Bug 995157!) or sync to the wrong profile (Bug 957131)… This is a great example of an ongoing complexity burden that makes every other feature more expensive to build and maintain. As you note, Lollipop (5.0, API 21) and onwards has an OS-level guest mode (which we don't always handle correctly, thanks to our own guest mode! Bug 1132387). The last time we discussed removing this feature was in 2015 (in my email archives). We added some more telemetry to make sure we had accurate measurements (Bug 1183724). We kept it around for a number of reasons, which can be summed up as "we weren't brave enough to call it a failed experiment yet": - It might have value without actually getting much use -- it's an occasional-use feature. (aka a "pet feature") - It demonstrates Firefox's commitment to user control. - Users might not be aware of it. But our "user control" campaign in 2015 apparently didn't make a dent, and if users don't know about it, it doesn't really demonstrate a commitment, does it? - It was baby steps for Kinderfox. - We were tentatively considering a Guest Mode for desktop, and more complex profile management on Android. IMO those arguments don't hold any water in 2017 -- it's a feature that people don't use, might not even be aware of, isn't supported on other platforms, has a better OS-level replacement on the majority of devices, and has significant engineering complexity.

I concur.

[triage] Bulk edit from title: this is a non-critical issue. Please remove priority if you wish this to be re-triaged.