Useful add-ons
I use a fair volume of add-ons and can never remember them all so I've put them here.
Firefox
xmarks (was foxmarks): good for bookmark + password syncing. I tried weave but never found it that good.
Ad-block Plus: keep spammy ads at bay.
No-script: The king of blockers really. Getting it to use a bookmark for the config combined with xmarks is sweet.
Long url please: Lets you see where those url shorteners are really pointing at inline.
Thunderbird
Quota display: I use IMAP for most of my mail so it makes sense to show my quota onscreen.
Addressbook synchronizer: I wish it were more like xmarks but for now having it sync to my FTP is plenty good enough.
Remember mismatched domains: Stops thunderbird complaining about certs that are signed by the wrong domain.
Ad-block Plus: keep spammy ads at bay in thunderbird too.