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 (126.96.36.199). I deleted all except this one & voila I had FDM working in Firefox ver 29