Manual Installation Steps for Firefox
It is necessary to have the steps for the manual installation of Firefox, since now it does not allow you to add any external extension without being verified, the most correct way for this is by teaching you how to activate the developer mode and then use the extension with the mode enabled.
https://support.mozilla.org/en-US/questions/1101877 normaly it is not possible to install plugins without being verified after firefox 44.
mybe this will help a little:
Firefox all editions (Temporary): about:debugging#/runtime/this-firefox > Load Temporary Add-on
Firefox Developer Edition (Permanent): about:config xpinstall.signatures.required = false > Install Add-on From File
It could be good to use github actions or something to create the VideoAdblockForTwitch-firefox.xpi archive.
It would simplify the installation procedure.