Firefox Quantum"Legacy" Add-Ons for Firefox Quantum (aka Firefox 57)

It's still possible to run 'legacy' (pre-Firefox 57 add-ons) on Firefox Quantum. First you can try to set the preference 'extensions.legacy.enabled' to true. But in most cases, the add-on will not run because the version information inside the add-on prevents that. But changing this version information will break the signing of the add-on, so it will still not run.

But there is another way! Unfortunately you have to redo the following after every update of Firefox. But well, if you really need the add-on... And i've successfully managed to install the following add-ons:

First you need the zip/unzip program from 'Info-Zip', version 3.0. This should be the standard zip program on linux. You can download the Windows version from Other zip programs like 7-zip might also work, but since Mozilla uses a somewhat different way to zip their files, that might fail.

Second, find your Firefox install folder. On Windows this might be "C:\Programs\Mozilla Firefox". Copy the .xpi files of your add-ons into the subfolder 'browser\features'.

Unzip the 'omni.ja' file from the 'browser' subfolder of the Firefox install folder (not from the main folder!) to a location of your choice:

unzip -qq "C:\Programs\Mozilla Firefox\browser\omni.ja" -d "somewhere\omni"

Navigate to "somewhere\omni\chrome\browser\content\browser" and open the file "built_in_addons.json" with a text editor like notepad. This file contains just one long line. At the end of the line, just before the ']}' insert the names of your add-ons (without the '.xpi'), separated by commas. In my case i change the line from

{"system": ["", "", "", "", "", "", "", "", "", ""]}


{"system": ["", "", "", "", "", "", "", "", "", "", "", "", ""]}

Save the file and be sure, no backupfile has left behind in this folder. Go back to the folder "somewhere\omni" and create a new omni.ja file:

zip -qr9XD ../omni.ja *

This will create the file omni.ja in the "somewhere" folder. If not already done, close Firefox. Copy the "somewhere\omni.ja" file into the 'browser' subfolder of the Firefox install folder (needs admin rights). Be sure no backup copy (even with another name) of the old omni.ja file exists in this folder!

Thats it. Start Firefox and enjoy!

Warning: Add-ons installed in this way do not appear in the add-ons page of firefox. From this it follows, that you can't access the options dialog of the add-on except the add-on implements a different way to access the options dialog.

Warning: To say it again: Every time Firefox is updated, you have to redo this procedure.


Some add-ons needs to be patched before they can be installed in this way. That is because some interfaces of Mozilla has been deleted or changed in Firefox 57 and the add-on has not been updated. This is the case for 'add bookmarks here'. But you can download a patched version from here. Right-click on the 'here' and choose 'Save as'!

29. Dezember 2017, G.Gersdorf