Well, technically it does work. With e10s activated the add-on still behaves exactly the same way as before. BUT: the browser itself becomes pretty much unusable. It takes very long for pages to load, and when they're finished, it still says "Connecting..." in the title. It can take several seconds to completely switch from one tab to another and then it gets rather messy. Only one tab at a time will be displayed properly, all other tabs become either black if the browser is maximized, or white if it is not. The elements are still there, i.e. the cursor changes its appearance when hovering above text or links, but they aren't shown and cannot be interacted with. Only when switching from maximized to windowed mode the current tab will be displayed properly and all other tabs will turn black/white instead. Also tabs cannot be closed. Other items like the menu or the bookmarks seem to work just fine. And one last thing regarding tile tabs: When I first tested it's functionality with e10s enabled it worked pretty well. After experimenting with the behaviour of tabs as described above I again tried to tile some tabs. Specifically, I was at wikipedia.org and wanted amazon.com to be above it. After a couple of seconds, in which the displayed tab flickered (sometimes showing both tabs, sometimes none), Nightly started opening new tabs. Then it asked if I wanted to stop a script that was executed by amazon.org, but it didn't help. I restarted Nightly in safe-mode, closed amazon and all blank tabs and restarted again. Since then, every time I start Nightly with both e10s and tile tabs active, it immediatly begins to open about one new tab per second. Ok, that is pretty much all I found until now. I might comment again when I solved the new-tab-issue.

Wow. That's pretty bad. Which Nightly version are you testing? Can you try resetting the browser.tabs.remote.autostart pref back to false, restarting Nightly, and then testing Tile Tabs in an e10s window (instead of full e10s browser): * open "File" menu > "Open e10s

... * "Open e10s Window" menu item The new window is running in a separate e10s process, but other windows do not. The full e10s browser (browser.tabs.remote.autostart = true) enables some add-on compatibility code that might cause some problems. Opening an e10s window skips the compatibility code, so we can isolate whether the problem is related.

Hm... I just tried to confirm this bug using a different machine, but this time the behaviour was quite different. When setting the whole browser to e10s like before, the add-on is actually working quite well, although it is still a little bit buggy in that changes in the tile-layout will only take effect after an additional mouseclick on one of the tabs. Also whenever I changed focus between tabs, the content area flickered shortly. In the single e10s-window, the behaviour is more like what I originally reported, but not quite the same. The browser is much more responsive in loading and changing tabs. But now tiling doesn't work at all. Even though the add-on states that the tabs are tiled, they aren't shown tiled. This time I used Windows 8.1 with Firefox Nightly: Version: 34.0a1 (2014-08-05) Build identifier: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:34.0) Gecko/20100101 Firefox/34.0 I will try to verify my original report tomorrow morning on my other machine and then also try using the single e10s window.

Alright, it seems the original bad behaviour isn't due to tile tabs alone. I also had Greasemonkey installed (though without any scripts) and these two add-ons together caused the unresponsiveness in a full e10s browser. After disabling Greasemonkey the behaviour is exactly as in my second report, it works, but it flickers. In the single e10s window, with or without Greasemonkey, the behaviour is also like my second report. Much more responsive, but tiling doesn't work. One more thing I noticed today: when the browser is unresponsive and the last tab to load shows about:newtab, than this new tab page will be displayed in all tabs. It can even be properly used. The version of Nightly is the same as yesterday: Build identifier: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:34.0) Gecko/20100101 Firefox/34.0

The developer has been contacted through AMO.

From the developer, here's the current status: alpha version (12.0a9) has been updated as far as possible and everything works, except for drag-and-drop operations (not implemented in Firefox Nightly) and thumbnails in Panorama (bug in Firefox Nightly). Not planning to release until drag-and-drop operations are available in Firefox Nightly.

The recent release seems to have fixed this. I may be missing something though.

Closing based on comment #8 and the fact that there have been multiple updates since. Please reopen if the problem can be reproduced.