I have a similar problem: ABP won't install on my Mac.
I tried blowing away the extensions.* files and restarting Firefox as suggested, to no avail.
ABP works fine in my main account (super user), but it won't install in the other user accounts.
Everything used to work with earlier versions of FF and ABP, roughly 6 months ago.
When I click on the "Install Adblock Plus 0.7.5.4" FF gives me a "Data Transfer Interrupted" message. I also tried to click on the en-US single language build. That actually downloads a file, but then FF gives an error message saying this: "Adblock Plus 0.7.5.4 could not be installed because it is not compatible with Firefox 3.0.3."
That clearly isn't possible, since it already works fine in my other account.
Thanks for any clues... I would like to have my kids protected once again from web advertising.