try tabmix on clean profile
I tried it on a clean profile but the problem re-emerged. However, as a result of this, I now believe the Session Manager extension is at fault. I say this as when I disabled that extension, and used the built-in TMP session restore feature- the problem did not manifest. Unfortunately the Session Manager extension is much more robust, so I will post information on that author's support forum in the hope a change can be made to allow the two extensions to co-exist and enable tabs to be displayed in the title bar.
A couple of notes regarding TMP...and I've used TMP forever and always have the latest Dev build:
1) Double clicking on the tab bar results in the window size changing (same as if you clicked the "restore down" button in the upper right of your screen). My desired behavior forever is for a new tab to open when I double click on the tab bar. I have only recently noticed this problem.
2) With respect to the TMP session manager feature...I found that even if I had the selection to "reload all tabs" unchecked, all the tabs would reload upon selecting a saved session from the TMP dialog box when starting FF.
Ironically, I identified this behavior a couple of weeks ago when using Session Manager and I notified that author. Here is his response:
Okay I've reproduced this. Apparently if a session is loaded when the browser is starting up and Firefox doesn't think it's a reload or a crash, the concurrent_tabs preference is ignored for the first window in the session. I'm not sure why that would be though since the concurrent_tabs preference is enforced by Firefox itself and Session Manager doesn't do anything different when restoring sessions at startup or when done manually later.
So this is technically a Firefox bug since it seems to be ignoring the concurrent_tabs preference at startup. I'm going to have to research this more, but I'm not sure I can fix this on my end.
I tracked the problem down to a bug in Firefox that causes it to think the number of loading tabs is -1.
It actually doesn't matter what session is loaded at start-up or if that session has pinned tabs or not. The problem is triggered by having pinned tabs at shutdown and it doesn't matter if Session Manager makes a back up session or not. The bug is triggered based on the number of tabs that were pinned on shutdown, the number of tabs in the loaded session and the browser.sessionstore.max_concurrent_tabs preference. The problem occurs on a per window basis. The problem doesn't occur if there were no pinned app tabs at shutdown.
With browser.sessionstore.max_concurrent_tabs preference set to 0, if the number of tabs in the session loaded minus the number of pinned app tabs is 2 or greater, than Firefox gets confused and will load all the tabs at once. This is on a per window basis.
So for example, if there is one app tab, then any session with 3 or more tabs in a window will have all the tabs in that window load simultaneously. If there is two app tabs, then any session with 4 or more tabs in a window will have all of them load.
With browser.sessionstore.max_concurrent_tabs set to a value greater than 0, it will simply cause extra tabs to load concurrently or sometimes not at all.
I've filled Bug 636279 documenting this problem.
The only way Session Manager could work around this is to delay the loading of the session at start up for a while which isn't really desirable.
I have the concurrent_tabs preference set to 0. I suspect this- or something similar- is causing the TMP session manager to fail similarly.