In newer Firefox versions 57+ legacy addons are not supported. So a way to use them in Firefox is to install them in the Nightly browser variation.
For this follow these steps:
2. Install but not run it.
3. Create a new Firefox profile for this version to prevent overlapping data with the default Firefox profile. If you want to use default profile for the Nightly version you may not create another one. But it depends on the versions of your default Firefox and the Nightly version. If the versions are identical you may use the same profile.
firefox -no-remote -p
4. Create a shortcut to the firefox.exe executable file, open its properties, in the Target field after the firefox.exe part add this text (parameters):
-no-remote -p [profile_name]
[profile_name] is the name of the profile you entered in the Profile manager in the previous step.
5. Run the link. It will open Firefox instance using the created profile.
7. Install the addon. If it’s available on the Addons website install it from there. But most probably the newest addon version will not be allowed on the website so you should have an .xpi file which is the addon itself. You need to drag this file to the Firefox window and allow its installation.
8. After it’s installed and is shown in the Add-ons Manager (Ctrl+Shift+A) enable it there.
10. After these operations there is a chance the addon will finally work. We’ll see what will happen with the upcoming browser versions…