To add to the confusion that Google has created themselves - there's
also a web browser called chromium, plus the core of Firefox is also
called chrome .... can't these folks be a little less obtuse with
their naming schemes ....
As for Firefox - the version in the repositories is old and in order
to 'run' the latest Firefox all you have to do is download the file(s)
for it from Firefox and run. There's no actual 'install' to do and it
will run from any folder - so you can have 5 / 6 / 7 / 8 all available
if you really want !!

FWIW the usual reason for Firefox to be troublesome is incompatible
add-ons and/or corrupted profile - and given that Firefox will look
for and use a profile folder then it's imperative to remove the old
profile completely and only transfer bookmarks to a new version.