Due to a conflict with Mozilla, the add-on's developer has taken it off the Firefox Marketplace.

type about:config into the address bar, press Enter, accept the warning, scroll down to xpinstall.signatures.required and double click on it to change 'Value' from True to False.

Firefox codec workaround on clear linux

chromium crippling adblock extension support (chrome is built on top of the open source project chromium).

good reason to switch to Firefox.

latest status as of 31dec19 is for the change to go live in 2020: https://www.reddit.com/r/uBlockOrigin/comments/ei0098/outcome_of_the_chrome_noadblockers_policy