In a comment, Mike Ray asked: “How did you resurrect TabMixPlus? This has been the bane of my online existence since I updated.”

Since this too was the bane of my existence, allow me to share in case any of you can’t wait for the developer to update.

1. In Firefox, go to Tools > Add-ons.
    
2. Uninstall the old version of TabMixPlus.
    
3. Reset Firefox.
    
4. Download the TabMixPlus dev-build, saving it to your desktop.
    
5. Using Firefox, at first I couldn’t figure out how to download an “add-on” without having it automatically install it. There may be some other way of doing this, but the easiest way I figured out was to download the file from Internet Explorer, which will let you just save it to your desktop.
    
6. Follow the directions mentioned in this thread by Ohmster. I’ve tweaked them a bit to make them a bit more clear:
    
1. Rename file to change the file extension (the three letters at the end, after the dot). Change .xpi to .zip.
    
1. If you don’t see the file extensions, goto Start > Settings > Control Panel > Folder Options.
    
2. Click on the View tab.
    
3. Uncheck Hide extensions for known file types.
    
2. Click to open with Winzip. Use the “Classic” interface.
    
3. Right click on install.rdf and Open With > Notepad.
    
4. Change maxver from 3.0 to 3.*
    
5. Save the notepad file. Winzip will ask if you want to update the archive, say yes.
    
6. Rename the file extension .zip back to .xpi.
    
7. Drag the file on top of Firefox and let go to install it. This will make it work with any version 3 of Firefox.
    

As someone in on the FF add-ons page said, Firefox without TabMixPlus is not Firefox.