This is likely tied to recent updates or experimental features in Firefox, especially in Nightly builds. These builds often introduce modern UI changes, like the Mica effect, but can also bring unexpected behaviors. In particular, enabling both vertical tabs and the sidebar hover expansion seems to cause the sidebar to expand over content or trigger tabs to open without input.
Some users have also reported rendering issues when the Mica effect is active for instance, the tab bar background might appear completely white. Temporary fixes like resizing the browser window or minimizing and restoring it can sometimes help by forcing Firefox to redraw the interface. Disabling the "Expand sidebar on hover" setting may also prevent the issue from occurring.
If you're using a custom setup or Firefox Nightly, switching to the stable version might help avoid these glitches. Advanced users have tried customizing Firefox further using userChrome.css to control sidebar behavior more precisely, but this approach requires additional setup and may not be for everyone.
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