latest Firefox upgrade
How do I get the previous version of Firefox back? I woke up this morning to a completely different website than when I went to bed last night. Whoever thinks this upgrade is an improvement is nuts. It took me 2 hours just to TRY to set it up as I had it including getting past the initial instructions for "refreshing" Firefox (no explanation provided for why it was even necessary), finding my bookmarked sites, setting up my preferences. Even though I have turned the popup blocker on, I lose the top quarter of my screen to an unwanted Walmart ad that I can't turn off. It follows me down the page, popping up in different places. There are also smaller ads that are unwanted, annoying, and impossible to block. And as for your "educated guesses", you guessed wrong. I'm a user not a techie. Right now the best solution seems to be deleting Mozilla products and moving to Chrome or Edge.
What page was displayed when you left the browser? What page was displayed when you returned?
As to the loss of your profile information, it sounds like the browser did a refresh.
Sometimes when the browser thinks there is a problem with the profile, it will create a new one.
Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.