Chrome
I suggest using this for devs or advanced users only. Ones that won't freak out completely if their browser crashes and they loose some data. This is solely because Text Area Cache is way better than Lazarus on Chrome.
See this post listing many of the Salesforce extensions Useful Google Chrome Extensions for Salesforce/
Users
- Lazarus
- Single Click for Google Apps (yeah, not a Salesforce extension but gives you back your missing black bar functionality in Gmail).
Admins
- Salesforce.com Sandbox Favicon
- Salesforce.com id Clipper
- Salesforce.com Enhanced Formula Editor
- Quick Login As - log in as other users in your Salesforce Org.
- Grey Tab - OMG, how cool is this app - never be without it. I always add "system" type fields to the page layout and clutter up the page layout. This tool just shows every field.
- It even shows the full HTML of rich text fields!
Advance Admins or Devs
- Salesforce Navigator
- See the excellent post Force.com Development Tips & Other Snippets by Matt Lacey: Flying Like A Pro: Salesforce Navigator for how to use Navigator.
- Web Developer
Firefox
I would use Chrome for admins or devs and use Firefox only for Users.
Users
- Australian Dictionary https://addons.mozilla.org/en-US/firefox/addon/english-australian-dictionary/
- Text Area Cache https://addons.mozilla.org/en-US/firefox/addon/textarea-cache/
- Snap Links
- Highlighter https://addons.mozilla.org/en-US/firefox/addon/highlightall/?src=search