In the not too distant past I was able to add custom search engines to Firefox in the search engine preferences (about:preferences#search). I don’t know when this was removed but do know that it’s not currently possible in Firefox 128.0.3 on Ubuntu 24.04. Note the lack of an “Add” button in the screenshot below.

A screenshot of the search engine preferences in Firefox

As per this Mozilla Bugzilla comment (from 4 years ago!) the solution appears to be to add browser.urlbar.update2.engineAliasRefresh and set it to true in Firefox’s Advanced Preferences (about:config).

A screenshot of the Firefox configuration option required to allow search engines to be added

With the above option set to true I’m now able to add custom search engines again.

A screenshot of the search engine preferences in Firefox after enabling them to be added

My motivation is to be able to add search engine shortcuts that aren’t (as far as I’m aware) possible using the techniques in the Add or remove a search engine in Firefox docs. For example I’ve added a shortcut to search wiki pages in one of our private GitHub repos:

A screenshot of adding a custom search engine in Firefox

Which allows me to type @gfrwiki in the Firefox Address bar in order to search for the pages I want instead of having to navigate to the wiki interface to use the in-page search.

A screenshot of searching the GFR Wiki custom search engine in Firefox