I've been clicking on the "Not now" button for the latest Firefox 3.0.1 security upgrade because I would lose the essential, to me, functions of Tab Mix Plus, which the upgrade disables.

-- I use "duplicate tab" all the time to keep a page open while I explore its links.

-- "Unclose tab" is a lifesaver when I accidentally close a blog item I'm writing -- it comes back up with all my work still there.

-- When I have many tabs open, often I've chased one topic for awhile; when I'm finished, I can "close right (or left) tabs" and a dozen or more pages close themselves.

This discussion, Firefox 3.0.1 Breaks Tab Mix Plus, at Justin, changed my mind. I learned that the upgrade is essential, and there is an upgrade for Tab Mix Plus that's compatible with FF 3.0.x; it's also getting some other changes, so it hasn't been officially submitted to Mozilla yet.

The pre-release Tab Mix Plus is here, and you can read about the kinds of things that are being fixed. Again, perhaps just to me, they're not essential -- "Close buttons on tabs are transparent in Mac platform," for example. As always, your mileage may vary.

If you're not yet using this extension, you can discover its wonders at its FF extension page and either wait for the official release or grab the upgrade in progress; it will automatically update when it's released, if Firefox is updating your extensions. Here's the blurb:

Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager with crash recovery that can save and restore combinations of opened tabs and windows

Tab Mix Plus has its own active discussion forums. It's that good.