How to install any old addon without this Rated 1 out of 5 stars
Here's a workaround for those who want any addon to be installed in FF versions later than x.xx
1-Download the addon xpi from any other browser.
2-Open archive inside any File unarchivers like 7zip,etc.
3-Locate the file install.rdf
4-rename it to install.txt
5-open it in notepad and find "em:maxVersion>x.x.*</em:maxVersion"
6-Replace x.x with 23.0,24.0,etc.
8-Rename it again to install.rdf
10-Drag the xpi file to the firefox window
11-Voila! Installation dialog box is here :)
Hope this works.