The short description of the problem is that the following link is broken and I'm not sure where I should go to download that file: https://addons.mozilla.org/en-GB/firefox/addon/disabled-add-on-fix-57-60/ A longer description is that KaiOS (mobile operating system going into use in places like india and africa) is building off of firefoxOS, and developing for this requires the WebIDE features that were in firefox until v59 (documentation here: https://developer.kaiostech.com/getting-started/env-setup/os-env-setup). Installing the adbhelper addon (https://ftp.mozilla.org/pub/labs/fxos-simulator/adb-helper/linux64/) in firefox v59 doesn't work because of an issue with an expired signing key. The workaround was apparently to install an addon (as described here for Firefox versions 57 – 60: https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/comment-page-5/), but it appears that this addon is no longer available. The KaiOS people might know of a workaround when I get in touch with them, but their documentation says to use this almost identical fix (similarly not available): https://addons.mozilla.org/pt-BR/firefox/addon/disabled-add-on-fix-52-56/