It is because you are using the Facebook Container add-on. This add-on limits the information that Facebook (and Facebook-owned companies, like Instagram) can access on Firefox. To do this, Facebook links will open in a special tab. This has the same result as opening a brand new tab because you will lose your history from your back button.

nosnalem said

On desktop, whenever I left-click on a link to a Facebook or Instagram page, the page opens in the same tab, but the top-left back button is grey (unavailable) as if I had opened a new tab. I noticed this problem maybe two weeks ago, it only happens in Firefox, and only with links to Facebook or Instagram.

Hi modinacomputer2015! Do you need help with something?