If your Firefox starts acting odd, slows down, hangs, crashes without reason, etc., chances are you could have a problematic extension installed. Sometimes these extensions are fine by themselves, but if paired with another extension could cause troubles. For example, running No Script and FlashBlock together can cause problems. Flash Blocking is included with No Script, so it is redundant to have both installed.
The folks over at mozillaZine and have come up with a list of known Problematic Extensions.
The list goes into details as to the issues caused by the said extension and offers solutions from changing settings to removing the extension and offering a substitute. Also includes references to the said problems.