Since I updated to Firefox 6 I have not been able to add new or replace old add-ons (firebug). When I click any add-on to download from the add-on panel I keep getting a download error message; when I am on the actual add-on page in firefox and click the install button I get a box at the address bar that reads "The add-on from addons.mozilla.org could not be installed because it does not match the add-on Firefox expected"
Some add-ons can produce this error if there is a problem with the calculated hash in the Add-ons site. This can be fixed on the site, but we need to know about it. Please send a message to firstname.lastname@example.org explaining the problem and the add-on that gives you this problem.
Some add-ons? I just tried to update two existing add-ons (FoxTab and RSS Ticker) and got the message and then tried to get some new add-ons (ColorZilla and Download Statusbar) to no avail.
If this is happening for all add-ons, then the problem is most likely not on the site. We would have certainly heard from many users if those very popular add-ons had incorrect hashes calculated for them.
It sounds like there's something corrupting your downloads and making the hash comparison fail.
Have you tried downloading the file (right click -> save as) and then dropping it directly in the Add-ons Manager?
Login as admin and install. Worked for me
edit: Actually, I had to install it as admin and then I was able to install it as a regular user as well.
I am able to install add-ons directly from the owners websites if they have them but not from Mozilla's add-on page.