Add-ons allow you to add extra features and functionality to Firefox, modify the Firefox user interface and change its appearance. Starting with Firefox version 57 released on November 14, 2017, only extensions built with this new technology will work in Firefox. Extensions created with the new standard are safer, more secure, and wont break in new Firefox releases. A majority of the most popular add-ons are already compatible, so most users should not notice any changes. Any incompatible extensions will be listed in the Legacy Extensions panel.”]
Source: https://support.mozilla.org/en-US/kb/firefox-add-technology-modernizing