In order to be able to use in a stable way an extension considered depreciated by Firefox, I was forced to reinstall version 54.0. However, it is not so easy to simply and permanently disable the updates.

To do this, it is necessary to create a personalised profile using the command : firefox -P and to systematically use this profile every time you use Firefox.

Once this profile is created and used by default, it is necessary to create a file named user.js in the latter’s directory which will contain the following lines:

user_pref("app.update.enabled", false);
user_pref("app.update.silent", false);
user_pref("app.update.silent", false);

This file will have the effect of overriding the settings included by default in Firefox to prevent it from updating itself automatically.

It can also be handy if you want to open Firefox with a script, to specify in the command line to use the particular profile:

/firefox -profile <path to profile>