For me my most essential ones (Firefox) are: uBlock Origin (advanced mode, RIP uMatrix), Right Click Search (lets you right click search using all available search engines), ClearURLs (mostly to make it a bit easier to copy the useful part of URLs, might help avoid some tracking), and SingleFile (downloads a page into one file using data URLs for images).
How? It is certainly not possible by default. You can set the right click search to be any particular one in the preferences but you can't, as far as I know, get a list so that you can search any particular text from any available search engine, which is what Right Click Search does. I did a quick search for "search" in about:config and didn't see anything that looked related.
