Are you using Firefox's Panorama feature? Mozilla designed it to provide Firefox users with better tab management capabilities. It allowed users to group tabs, only display groups in the tabbar and display all tabs in full screen on a separate page.
I ran into issues with the feature when it was first introduced back in 2010 because if the shortcut key that the feature was mapped to and what was likely a bug in Panorama.
What made things worse for me was that there was no option to disable Panorama. It was one of those things where the developers decided that Panorama was good and everyone had to live with it regardless of whether it is used or not. You can invoke Panorama currently with the Ctrl-Shift-E shortcut.
A recent bug listing on Bugzilla indicates that Mozilla plans to remove Panorama code from the Firefox web browser, move that code into an add-on and offer that add-on instead to users who are interested in the features Panorama makes available
Panorama has a very small user base and there's basically no chance that we will enable it by default or even improve its visibility and discoverability anytime soon. In its current form it is indeed quite fragile and causes intermittent test failures here and there when touching platform code. We're not going to spending any time on improving Panorama and its feature or its design and UX - thus it should just be removed from Firefox.
I myself still am a Panorama user and I don't think there's any replacement for it so I think the way to go is to provide an add-on that provides exactly the same feature. It is my belief that this will encourage contributors to help keep Panorama alive because it's much easier to contribute to a self-contained add-on.
I prepared a patch to remove Panorama from Firefox and moved all the code an add-on that provides the same functionality. The add-on does of course only work with the patch applied to a custom build.
Mozilla plans to install the Panorama extension automatically for existing Panorama users so that they ideally won't notice any difference and can continue using their tab groups.
I'm not using Panorama at all and won't really miss the feature. While I would have been ok with an on / off switch as well, turning the feature into an add-on is even a better solution as it will remove code from the browser that the majority of users do not make use of. Those who do can install the add-on or get the add-on automatically to continue using it. (via)