There are literally hundreds of add ons for you to chose from. Everything from security enhancements (NoScript) to an online dictionary (not used too much by me as Firefox checks spelling as you type) to addons that make use of this forum and bb forums much easier (BBCode, BBcodeXtra). The latter two make posting URL's, images, italics, etc. as simple as a keystroke. But my advice is not to load up with too many as they could cause a problem interacting and maybe a crash.
Thank you for that.
Those wanting to have a much more pleasurable Freerepublic reading and posting experience should also check out the Firefox Greasemonkey plugin with the FR Posting Form Enhancer and FR Tree Viewer scripts.