Well download the add on from chrome because you will know where you have downloaded it.

It is an .xpi file, so simply rename it to .zip instead of .xpi and you will see an archive. Extract it and make the changes in the folder extracted.

Then again Compress the folder back to .zip and not .rar.

Now change the extension back to .xpi and now drag and drop the .xpi into your firefox and wallah the add-on is installed on the browser back

pi0tr3k stated the same but I guess I wrote a lot