Some of my favorite ones that I found and use.

I you do a lot of searches this really is a good one to have..

WOT {web of trust}- https://addons.mozilla.org/en-US/firefox/addon/3456
[WOT, Web of Trust, warns you about risky websites that try to scam visitors, deliver malware or send spam. Protect your computer against online threats by using WOT as your front-line layer of protection when browsing or searching in unfamiliar territory. WOT's color-coded icons show you ratings for 21 million websites - green to go, yellow for caution and red to stop – helping you avoid the dangerous sites. Surf safer and add WOT to your Firefox now
Keep yourself safe from online threats such as:

• Spyware, adware and viruses
• Browser exploits
• Unreliable online shops
• Phishing, spam and other Internet scams
• Annoying or malicious spam

If you are about to enter a risky website, WOT will warn you and save your computer before you interact with a harmful site. With safety ratings of 21 million websites, WOT combines evidence collected from multiple trusted sources, like phishing and spam blacklists, with the ratings provided by WOT users. The system uses sophisticated algorithms to produce reliable and up-to-date ratings.

WOT reputation ratings have four components: ] see link for more details/info

WOT IE addon
Keep your computer safe from online threats like spyware, adware and viruses. Get warnings about online scams, sites with adult content and spam. Protect your wallet from unreliable online vendors, phishing attempts and identity fraud.

Join us now and download the WOT addon for Internet Explorer. It's free!
http://www.mywot.com/en/download/ie



AnyColor - https://addons.mozilla.org/en-US/firefox/addon/6991
you can change the colors of the default FF theme , customize it with colors of you own choice {sliders for tint, brightness etc}, or use a photo as the background image for the top, side and bottom.
11 set choices or you can choose random, or make your own custom set up.
also font size adjustment.

If the standard gray is too bright for your eyes and you haven't found a theme you like this is just what you will like.
You can make changes on the fly -while still using the browser
want blues change it, purples , bright , dark whatever you like.. whenever you want a change.



Forecastfox - https://addons.mozilla.org/en-US/firefox/addon/398
[Get international weather forecasts from AccuWeather.com, and display it in any toolbar or statusbar with this highly customizable and unobtrusive extension.]
show weather for any zip code or location you choose and you can make more customizations in the options section.
I have a 4 day forecast with large icon set
more info and icon packs -
http://forecastfox.mozdev.org/