It's fascinating to discover many useful extensions for Firefox and Thunderbird, which offer unparalleled user experience. Here I'd like to share some examples that I'm using. First please see a list of add-ons for Firefox.

1)
Adblock Plus: the web page is much cleaner without the ads.
2)
Add-on Compatibility Reporter3)
All-in-One Sidebar: It appears to be incompatible with the default theme when switching the themes, and the
development build has the fix.
4)
Chromin Frame: bringing the same look and feel as Google Chrome browser.
5)
Chromin Tab Behavior6)
CoolPreviews7)
Coral IE Tab: Originally I was using the
IE Tab which is not compatible with the new Firefox 3.6; then I switched to the Coral IE Tab. You need to check the developer's web site for the
latest add-on.
8)
Dafizilla ViewSourceWith9)
Disablemenu10)
Download StatusBar11)
DownloadHelper: quite handy to use when downloading videos.
12)
Fission: Fission combines address bar and progress bar (Safari style). This makes the progress bar more visible and allows for a nice visual effect.
13)
FoxTab14)
Google Voice Add-on for Firefox: very neat add-on to recognize the phone number on the page and you can click to dial the number via Google voice.
15)
gTranslate16)
Locationbar²17)
Map with Google: Originally I was using the
Map This extension, but it was no longer working after I installed
gTranslate; then I switched to using
Map with Google.
18)
New Tab King19)
Organize Status Bar: you'll need it as the number of icons are going in the status bar.
20)
PluginChecker: Check at application startup if there are plugins installed without user permission
21)
Print/Print Preview22)
QuickDrag23)
Tab History24)
Toolbar Buttons25)
User Agent Switcher26)
WebMail Notifier27)
Xmarks
The theme I use for Firefox is the
Strata RELOADED.

Then for Thunderbird, I'm using the following add-ons:
1)
CompactHeader2)
Display Mail User Agent3)
Forward4)
Hide Menubar5)
Lightning: The Lightning 0.9 is not compatible with Thunderbird 3.0, and I have to use the
nightly build.
6)
Toolbar ButtonsThe theme for Thunderbird is the
OWN.