Solution: I came up with this uBlock Origin filter rule: startpage.com$removeparam=/abp|lui/, I started by removing all the paramters and then took them out one by one while testing if searches made on startpage.com worked with them: startpage.com$removeparam=/abp|t|lui|sc|cat/. I also got a response from u/SPSupport that the paramters don’t show when I search from the URL bar with my search engine set to https://www.startpage.com/sp/search?query=%s, which I did.
I want to be able to see the search query in the URL, and have a URL that I can always go back to if I restart my browser. I set the HTTP request method in settings to GET instead of POST since that shows the query, but it adds a bunch of extra paramters:
This is how GET works, every single parameter is visible regardless of meaning.
How about https://addons.mozilla.org/en-US/firefox/addon/clearurls/ ? I use it on default settings. There is a chance you can set it up to clear StarPage’s GET variables too.
But if there is an actual setting to use exclusively GET parameters by StartPage, they will just be populated again.