Problem:
if there is a firefox instance already running, the logic in webhttrack will
never terminate.

webhttrack starts a new firefox window and checks if firefox is running. If
you close only one of the two windows the script continue to loop ...

as a possible solution:
xwininfo -name 'HTTrack Website Copier - Offline Browser - Mozilla Firefox' |
grep -iv 'No Window' | wc -l

results in 0 if the firefox window for httrack is closed and in != 0, if
httrack is still working.

So this could be a better test to check for the termination of the firefox
instance ?
The problem is to figure out all possible combinations of text strings for
different languages and/o browsers. But perhaps you can launch firefox with a
special string, equal for all combinations ?
Keep up your good work on your great tool !