TLDR: FF27.0.x + FEBE 126.96.36.199 causes text (but not icons) in FF tabs in additional FF windows to vanish after FF restart. Solution: upgrade to FEBE 8.0 beta. Do not blame Tab Mix Plus. Update: see below.
Long version: After upgrading to Firefox 27.0 (and FF 27.0.1) I began to experience an annoying bug. I am in the habit of having a lot of tabs open at once. Thanks to Tab Mix Plus, one of my very favorite addons, I can have the tabs arranged in multiple rows and they stay big enough to have an idea of which is which.
I also tend to have at least two firefox windows open, one for each monitor. I also use the session manager in TM+ to restore my tabs when I close and re-open firefox.
After the upgrade to FF 27.0, I found that the text was vanishing from the tabs when I did a re-open. The icons were there, but the rest was blank. This didn’t happen in the first window I opened, but it was happening in the second window, and every new window. The problem didn’t happen if I closed all the tabs before shutting down firefox. It did happen whether I used TM+’s session manager of FF’s native session manager (yes, I even if I unchecked the setting the in FF privacy manager that tells it to forget my browsing history).
I figured this was a Tab Mix Plus issue. The extension is so powerful that it regularly has issues when I update Firefox. But his time, disabling TM+ didn’t solve the problem.
So it was time to start disabling all my many other extensions in the hope of finding the culprit. This is tedious, even doing half at at time, but it did reveal that the source of my problem was FEBE, the Firefox Environment Backup Extension. Upgrading to FEBE version 8.0beta solved the problem. The beta actually looks a lot better than the old version (the author says it is a complete rewrite and I believe it). It seems noticeably faster, and (hooray!) it allows you to add incompatible add-ons to the ignore list from an interactive dialog as the backup is happening, rather than having to go to the preferences after the backup is over.
If you are updating FEBE from an old version be sure to delete existing FEBE preferences before the update. Tools > FEBE > FEBE Options > Advanced > Clear FEBE preferences.
After the install, whether or not this a new install, you MUST go to Tools > FEBE > FEBE Options. The only option that absolutely must be set is your Backup destination directory under the Where to backup tab. Without this it will not work.
Update: Having tried to fix this on a second computer, I have to agree with commentator parabel that in fact TM+ is involved in some way. With FEBE updated to the beta, I was only able to get a second window to open properly — but not a third. Disabling TM+ solves the problem, but I lose all the TM+ functionality.
Update 2: As noted by commentator John, installing Tab Mix Plus version 188.8.131.52 seems to fix the problem. Yay!