I-Firefox Isithangami Sokwesekwa

This randomly happened over the past few months but last couple days it's bad. The Google Sheets cells I have open on Firefox turn black. You can scroll and they will come back but just briefly and back to black. Very annoying as these are shared sheets for my business with other employees. It's not happening in Vivaldi Browser or Edge. I hate Chrome so not even trying it there but guessing it's fine. Seems to just be Firefox all of a sudden. See pic. Help please as I have no clue what's causing this.

I have made Firefox my default browser, and set Google as my default search engine.

I'd like to keep it that way, BUT ...

Every morning when I come to work and open my browser and begin to search .. Yahoo comes up .. or Bing ???? I delete them, and again set Google as the default ... BUT FIREFOX CONTINUES TO REVERT TO YAHOO, or BING SEARCH RESULTS  ?!?!?!?!?!?

Even when I deliberately open Google and search in Google .. the results appear and then suddenly, a new window pops up with Yahoo results, covering up the Google results????

How can I make Firefox respond to the default search setting I choose????

This is intolerable. If I can't get ride of Yahoo and Bing and all these other inferior search engines, then I'm going to go back to using Chrome.