Oh yes, I did just go there. Hands-down, without a skerrick of doubt, AutoCopy is the best Firefox extension. It may also be the best Firefox extension you've never heard of. Here's what it does, and then I'll tell you what makes it so great.
Developed at Mozilla, AutoCopy is a lightweight, single-feature add-on that copies any text you highlight to your clipboard. No more hitting CTRL+C, or using the context menu. That in and of itself is not so revolutionary. The feature has been around for a while in other programs. What makes it the must-have extension is that there's practically no other reason to highlight text on a Web page except to copy it to your clipboard.
Sure, highlighting can be used to reveal hidden words or perhaps make poorly-colored text stand out from a background, but those instances are few and far between. If they're not, you're spending too much time looking at badly designed sites. To do either of those when using AutoCopy, just hold down the CTRL key as you highlight and it won't copy it to the clipboard.
Once you've highlighted anything from a single letter to entire multipage New Yorker articles, the add-on opens a small options box where your cursor is. Through the extension options, you can configure how long that box appears for, or turn it off.
If you choose to use it, the post-copy options box offers a couple of useful choices. You can undo the copy, or access up to 10 previous clipboards and bring them back as the active clipboard. You can also paste to the location bar or the search bar, search from your default provider using the clipboard text as the search term, or open the text in a new tab. This doesn't use the "feeling lucky" search, so it only works for URLs or FTP sites. The last option copies the URL to the clipboard.
Options to configure add-on behavior include toggling a status bar icon for the add-on options, paste on middle click, deselecting after you highlight, toggling AutoCopy in text boxes, blinking to notify you when it copies, and copying plain text. That last one requires an additional extension, and I found it to be more than I needed.
Back in 2007, my colleague Peter Butler thought that Tab Mix Plus was the best Firefox extension, and I agree that it's still an excellent one. If you're using the pre-release version of Firefox 3.5, you can grab a beta of the updated Tab Mix Plus here. Tab Mix Plus isn't for everybody--as he says, not everyone needs to make all of their tabbed browsing dreams come true. Not everybody cares about in-page ad-blocking, either. Copying text, though, is something everybody does in-browser, and it'd be great to see this functionality eventually built into Firefox or one of the other top browsers.