Hi   !

Switching to Nightly might be an option, see :

https://www.ghacks.net/2017/08/08/firefox-your-options-to-run-legacy-add-ons/

and also :

https://www.ghacks.net/2017/08/12/how-to-enable-legacy-extensions-in-firefox-57/

This article may also be helpful :

https://support.mozilla.org/en-US/kb/unable-install-add-ons-extensions-or-themes

hi, the proper fix here is to upgrade to the latest version of IDM, which works fine in firefox - we had to blocklist the old version because it will cause non-stop crashing in firefox 53 and above.

Just in case you're interested : When you look at this article and scroll down, you'll get a list of add-ons with most users, top-rated and featured add-ons and their current status :

https://www.ghacks.net/2017/07/07/firefox-add-ons-webextensions-state-july-2017/

And here you will see add-ons listed that are compatible with Firefox 57 :

https://addons.mozilla.org/en-US/firefox/tag/firefox57