In addition to Flatiron suggestions if you did just a upgrade try this.
Go to your Firefox Profiles at C/User/Your User Name/AppData/Roaming/Mozilla/Firefox/Profiles/Your Profile/Extensions and look for the folder for Free Download Manger.
There should be a file called install.rdf in that folder. Open it in a text editor Notepad++ Home
Now you need to change the maxversion to 11. Find this line
.0.*</em:maxVersion> (the 3
could be a higher number is just an example)
Save the file and close the editor.
Restart Firefox is see if extension will work now.
You will get a window that will open up in Firefox asking you about this extension just click okay or something like that.