Greasemonkey, a website customization scripting engine, is available at Mozilla add-ons and the official website for the extension is at greasespot.net. A good extension, for Greasemonkey itself, is Greasefire, which automatically searches, on a per-site basis, GS scripts on userscripts.org.
I've been using it for some years now and I often forget that I can actually use it to fix gripes that I have! Perhaps this thread will alleviate that a bit - often, it's really just a matter of googling for what you need.
A preliminary list of suggestions:
- Blogger Content Warning Autoskip II for Greasemonkey, disables warning
- MSPLinks for Greasemonkey, rewrites MySpace links back to normal
- deviantart outgoing link repair for Greasemonkey
- facebook outgoing link repair for Greasemonkey
- MySpace Comment Hider for Greasemonkey