Adobe's Flash plug-in has grown more robust over time, but it's still one of the major causes of instability in Firefox. You could always avoid sites containing Flash content, but that's not exactly practical. Instead, you can try and minimise where things can go wrong by tweaking your browser's settings and performing maintenance on your system.
Firstly, you can make sure you're running the most recent version of Flash via Plugin Check, a service provided by Mozilla that streamlines the update process. Your next step should then be to set Flash to "Ask to Activate", which prevents Flash content from running until you click on it.
Along with these suggestions, gHack's Martin Brinkmann recommends using two browsers, with one set up for Flash content and the other regular web activities, though this can be problematic as it's not always easy to tell if a site is using Flash. You can also disable hardware acceleration and protected mode, though the latter should be a last resort.
Brinkmann offers a few more tips, which you can read in full over at gHacks.