To install Angular DevTools in the firefox browser. Follow below steps:-
Open menu item from top bar Tools -> Browser Tools -> Remote Debugging.
After clicking Remote Debugging click on This Firefox option from left navigation. You will see the below screen with list of extensions. Temporary extensions are unreleased extensions loaded in the dev mode.
Click on Load Temporary Add-on button to select and load Angular Devtools extension for firefox.
Select any file from dist -> shell chrome directory to load extension. If you have not build extension for firefox yet. You can do it using yarn build:firefox which will generate build for firefox.
After selecting file and clicking open, you should be able to see Angular DevTools as a Temporary Extension in firefox.
Open any angular application in dev mode and open Firefox Devtools you should see Angular Tab in there.