Much as I love Firefox 3, several of my favorite extensions still haven't been updated to support the new version of the browser. Chief among them is PermaTabs, which lets you make any tab "temporarily permanent" (meaning it appears each time you run Firefox).
Thankfully, a coder by the name of Deos decided not to wait for the original developer to update PermaTabs, so he whipped up PermaTabs Mod. It functions just like its namesake, but works in Firefox 3. Happy day!
After installing the extension (which is free, of course), you can make any open tab "permanent" by hitting Alt-P. (You can also right-click a tab and choose Permanent Tab from the context menu.) Said tabs turn yellow so they're easily distinguished from regular ones, though you can choose any color by dipping into the Options menu.
Honestly, I'd be lost without this extension. I use it as a kind of temporary-bookmark feature, so I can easily return to sites I want to revisit in my next session. It's a must-have for Firefox users.