Far and away my favorite Firefox extension is
Lazarus.

I can't tell you how often I've been typing something (a blog, a request for tech support, a forum message, email, etc.) when suddenly I lose my connection, or I submit the message and it disappears into thin air. Thanks to Lazarus, I no longer worry, at least not if I'm in Firefox: I simply click on the tiny Lazarus icon at the bottom of my Firefox screen, find whatever I was writing safely stored in its entirety, and retrieve it. Lazarus stores all entries for an adjustable amount of time, so I can go back and find something I typed a couple of days ago or even last week. Lazarus has saved my bacon
many times; I've been so grateful that I've donated money to the developer as a way to thank him for this terrific free add-on. I should note that Lazarus is now also available in a beta version for the Chrome browser.