no thats the directory, in llinux sometimes a directory will start with a "."
try ls -al in your home directory from a terminal and you will see that all your config files will be listed and they will all start with a "."
in this case though a directory is created in /tmp by firefox and im guessing its renamed each time, so if you go to /tmp and try ls *.tmp it will show up

, the firefox temp files will be INSIDE that directory, like I said mine is in /tmp/sv001.tmp
yes you can delete the temp files if you want, but your system should do it on its own periodically
just navigate there and take a look, isnt hard, and it can be done while its running