I would uninstall it completely, reboot, run something like ccleaner to remove any registry orphans, etc.

Now reinstall the latest FF version and don’t go berserk on add-ons, start with your security ones first like, NoScript, RequestPolicy, AdBlockPlus, etc. See what that is like and then add your most frequently used add-ons a couple at a time and see if there is any impact. If so you only have a couple of add-ons to test (disable or uninstall) one at a time to find the culprit.

Don’t install add-ons from untrusted sources.

For me firefox starts up in around 8 seconds for first start and I have 27 add-ons and use a theme also.