Why does Firefox add tracking parameters to Google searches?
It appears to add fewer than other browsers, but I'm not quite sure why it adds "?channel=fenc&client=firefox-b-1-d" to searches when only the q/query parameter is needed for them to work.
Google pays Mozilla to be the default search engine.
My guess is that this is part of that deal, so they can track its effects: this way Google can see how many Firefox users arrive at a Google search through that search integration (as opposed to via other paths like clicking links or entering the URL manually).
Probably in order for Google to know how much of its searches come from being the default search engine in Firefox clients. It is probably part of their deal, and would logically affect the amount of money Google is paying Mozilla to be the default.