Thunderbird: Upgrading While Keeping Incompatible Extensions

I was using Thunderbird 3.1.16.  I wanted to upgrade to the current version, which was Thunderbird 8.0.  In 3.1.16, I was using two add-ons that, I feared, would not be compatible with 8.0.  Those two extensions were ImportExportTools and Remove Duplicate Messages.  (I also had AttachmentExtractor, but it was apparently nonfunctioning.)  I had found a MozillaZine webpage that suggested a way to make extensions work even if they were supposedly incompatible.  So now I decided to see if that advice would work for me.

I started by installing T-bird 8.0.  During the installation process, it asked me if I wanted to keep my add-ons.  I said yes.  When the installation was done, Thunderbird started.  I went into Tools > Add-ons.  This opened the Add-ons Manager.  It displayed all three add-ons as being ready to go. 

I don't recall what happened next.  Apparently I didn't need those extensions for a couple of months.  When I returned to them, I found that they were labeled as incompatible with Thunderbird 8.0.  But when I upgraded to T-bird 10.0, it looked like they were all set to work.  I used Remove Duplicate Messages (Alternate), and it seemed to work.  End of issue, at least for now.