> HomeDepot.com often won't let me add things to cart. The past couple months it has been working again, but there were a few months where I couldn't.
I checked for webcompat issues matching this description and couldn't find anyone reporting that adding items to the cart didn't work.
I attempted to reproduce the issue myself, but HomeDepot block visitors from my location.
> This morning, I couldn't buy a key off cdkeys.com - tried two totally different methods, email addresses, credit cards. Didn't try a second browser yet but I'm not sure I am that persistent...
Can't reproduce, I was able to buy a CD Key ("Garfield Kart - Furious Racing PC") at 28 May 2022, 16:51:52 BST using Firefox 100.0.2 and a debit card.
> A news site, I think the New Yorker, would only show the first two paragraphs of an article but in other browsers, would show the full article.
Can't reproduce. Visiting https://www.newyorker.com/news/daily-comment/the-baby-formul... in Firefox shows me the exact same content as when I use Chromium.
I'm not saying you've not experienced these issues, but they're definitely not widespread and your post doesn't give enough information to go on to diagnose if there's an issue with Firefox or not.
The best thing you can do is report issues to the webcompat project as and when you come across compatibility issues. It takes about 20 seconds, less time than you probably spent writing this comment.