chattr +a -V

A comment on the Curious George: Firefox recent addresses post at Monkeyfilter lead me first to Miscellaneous Firefox Tips, and from there, to The Extensions Mirror, which has 402 Firefox extensions not available on update.mozilla.org. The link lists the extensions ascending alphabetically by name. I didn't see a way to sort the extensions by date or a crude measure of popularity (such as number of views or downloads). I don't use the Thunderbird email program, so the 65 Thunderbird extensions not available on update.mozilla.org don't interest me.