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:

1. Download Nightly installer from nightly.mozilla.org for the latest version of from the FTP for previous versions.

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.

To create a profile:

- press Win+R keys

- type

firefox -no-remote -p

- in the window press Create Profile

- enter any profile name

- Finish, close the window

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.

- to allow legacy addons: load the next URL and set the preference to true

about:config?filter=extensions.legacy.enabled

- to allow unsigned addons: load the next URL and set it to false

about:config?filter=xpinstall.signatures.required

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.

- to open a non-multiprocess window go to File – New Non-e10s Window

- to disable multiprocess in the browser go to Options and uncheck ‘Enable multi-process Nightly’

- or disable it through the about:config page: load the next URL and set the preference to false

about:config?filter=browser.tabs.remote.autostart

10. After these operations there is a chance the addon will finally work. We’ll see what will happen with the upcoming browser versions…