Description of problem:
Already open tab name are not translated when you change language.
Version-Release number of selected component (if applicable):
Steps to reproduce:
1 - Open one or more new tab (tested in english, french and japanese)
2 - Change language
3 - Logoff as asked
4 - Open firefox and restore page if needed
Previous new tab name are still in old language
Previous new tab name to be in new language
If you click on the previous new tab, the name of this tab change to the good language.
If you mean a tab named "New Tab" (ie. without any loaded page) we can't do much about it. Last tabs are restored from profile_dir/sessionstore-backup/previous.js file where the "New Tab" string is stored along with tab url. That makes loading list of last opened tabs very fast.
The downside is there does not do any translation during loading, but since frequent changing language for one's profile is minor use case therefore checking language changes each time Firefox is launched and rebuild tabs list would be making things slow, thus closing as won't fix.
If you meant different issue, please reopen and elaborate with more detail.