I'd not recommend copying the entire profile contents from the
troublesome Firefox back into the new install - the problems you are
having are possibly from corruption in the profile so copying back
will just put you back to square one

The only thing I'd copy back is the bookmarks file !!

As for removal / re-install - go to [ System / Administration /
Synaptic Package Manager ] and enter Firefox into the search box then
right click on it and select 'Mark for complete removal' then click on
the 'Apply' button. Following that, tick Firefox and click on 'Apply'
once more to re-install cleanly.

Now start Firefox - this is important to initialize the profile !!

Next, close down Firefox and copy the saved bookmarks file from the
old profile into the new one. Re-start Firefox and re-install the
add-ons you used to have. Just a point here - try not to overload with
lots of these, only put back what you *really* need as the one thing
that can cause major problems with Firefox is having too many, or
conflicting, add-ons installed !! Remember that these are all created
as stand-alone add-ons and nobody can possibly test against every
other add-on in existence !!

My FF has never once given trouble on this ubuntu.