archived-content VS add-custom-search-engine

Yes we can still create uc.js in 2022. MDN no longs serve the XUL doc. However we can search their archived doc repo, e.g. https://github.com/mdn/archived-content/search?q=nsIPrefServ...

MDN used to have some good documentation, back before webextensions when it was expected that third-party addon developers would be modifying the frontend directly. that's no longer supported though, so all those juicy pages on XUL and XPCOM have been removed. they can still be found on here but not in a very useful format.

it's archived on here but the pictures are missing so it's just... fucked honestly. I am so frustrated and aggravated by this. it makes it that much harder for someone to pick this up as a hobby, which makes it that much harder for firefox to find new contributors. anyway, frame scripts are a bit annoying to use. but since about:newtab is a firefox system page (it has a system principal), addons can't inject anything into it either. so you couldn't use greasemonkey or something.

The page source can be found at the mdn-archive

It’s a kinda workaround. There is an addon - https://addons.mozilla.org/en-US/firefox/addon/add-custom-search-engine/

Add Custom Search Engine Because people don't support OpenSearch :(

I'm attempting to switch back to Firefox from Chrome. It seems like the ability to customize search engines in FF is kind of broken. Yes adding an "open search" site is supported but adding other search engines manually requires requires a special plugin or special settings (see also this bug report).

You said that you don't want to install anything, but if you do install this extension, you can easily do this: Add custom search engine. You can uninstall it afterwards. It was developed by someone who works for Mozilla.