I could be crazy, but I think this behavior has recently changed in Firefox. I often open links in Facebook (or Reddit, or Twitter, etc), which opens a new tab to the right of the (origin website's) tab. When I am finished reading the link, I will use CTRL+W to close that tab, and it used to switch back to the previous tab from which I clicked the link. In other words, if I open a link in a new tab, when I closed that new tab it switches back to the tab where I clicked the link from (assuming I navigate to no other tabs or websites).

However, the behavior now is different in where Firefox will switch to the /next/ tab on the right, instead of back one to the left. This adds extra steps to navigating websites which have a lot of external links, such as any social media platform, because I have to manually switch back to the website's (Facebook) tab after closing the new tab.