If you are a regular Firefox user AND also a Free Download Manager user, you would have definitely
encountered that after updating Firefox to the latest version the FDM plugin
might have got disabled (with no enable option) with a message that it’s not compatible with your
current version of Firefox. This is because FDM doesn't delete the
old extension directories so If this happens then do the following.
Go to ‘C:\ProgramData\Free
Download Manager\Firefox\Extensions’ and delete the folders which have
the number other than your current ver number.
In my case I had 5 folders other than the current (1.6.0.3).
I deleted all except this one & voila I had FDM working in Firefox ver 29
