Vital - but where is it's cache? Rated 4 out of 5 stars
For me Tree Style Tab is a vital Firefox Add-In. e.g. currently I have 5 windows and 147 tabs.
... but in recent months I have seen problems in the case of a rare Firefox crash (which I don't suspect Tree Style Tabs of causing)... when restarting Firefox after the crash it the Tabs do not properly reload. Creating new tabs may not add a tab to the tab bar. A tab tree may have the icon indicating it is expanded but it is closed. On 'closing' it it might actually open.
I presume some cache needs emptying or resetting but I do not know where to look. I usually manage to fix it after a few restarts, perhaps closing all my tabs (I can get them back later from Session Manager), and disabling and re-enabling, or even un-installing and re-installing Tree Style Tabs.