First get the GreaseMonkey Firefox extension.
Then write your own script or install some of the hundreds available at UserScripts.org.
Here are my recommendations:
User Scripts Search Sorting - With Install Rate improves the sorting on userscripts.org making it easier to find scripts. Use it to toggle between 'install rate' or 'installs'
After a secure login gmail defaults to HTTP so I got tired of manually switching it back into HTTPS. Secure-em-All automagically secures many webmail sites.
Sourceforge Direct Links saves time with direct download links from sourceforge projects. But save yourself some time and do not install either cleaner layout scripts because they no longer work.
Download Video allows you to download videos from youtube, gvideo, and myspace.
Textarea Backup w/Restore Buttons automatically backs up the text you type so you'll never accidentally loose another email. It has a restore button so it's better than the script that automatically restores text [when you do not want it to].
You'll find tons more for the sites you use and dont worry you can turn greasemonkey or any script on/off with a click of a button.
View the source before you install each script just to verify it doesnt do anything sketch and I hope you find it as useful as I do.