That's strange. Does it show the page saying it couldn't restore automatically and asking whether you want to restore or start a new session?

on 3 tests (thanks to my unstable gpu)

if it restore without asking the tab name is normal if i quit then restore via history->restore the tab name is normal

when it ask to restore or not, then tab name is not ok

Okay, I haven't figured out how to replicate that.

It sounds like the starting point is the about:sessionrestore page, which appears when Firefox not able to do an automatic crash recovery. You can confirm the internal URL using the Page Info dialog - Ctrl+i - if it's not in the address bar.

Does that page get partially/unsuccessfully replaced by a restored page, or is it simply left there instead of being closed?

yes, when it show the about:sessionrestore that page get replaced by the first tab of the old session but not the name of the tab. even when doing F5, the content get reloaded but the tab name stay "session restore" (well actually i got french language pack so it's "restauration de session")

That sounds like a bug. You can file a bug report at:

https://bugzilla.mozilla.org/

The form is a little bit arcane, but this article covers the important points:

https://bugzilla.mozilla.org/page.cgi?id=bug-writing.html