Couple of things:
1. It might help to use http
s:// (note the "
s") instead of the old http:// (note lack of an "
s"). But the weds/browsers are pretty good at adding that in their DNS trips. Usually, you don't even need to add the "https://" to just the sites name.
2. Look for a "History" item in Firefoxes menu. There should be a "Clear Recent History..." item in there. Use that and clear out the recent sites you've visited.
3. As long as you are in Firefox, make sure you have the latest version that will run on your Mac. Looks like I have a 64-bit version: 126.0.3 (64-bit). Your Mac may not use 64-bit CPU's. Just use the "About Firefox" (under the "Firefox" menu) and use any "Restart to update" button that may be in the About window. There should be a "Firefox Updates" section in the Settings... (Preferences).
Firefox 48 was released on August 2, 2016. ... It is the last Firefox version to support Mac OS X Snow Leopard, Mac OS X Lion, and OS X Mountain Lion.[128]
Just for grinz, how are you trying to get into a TS Preview display?
I am assuming that there is an actual "Preview" button next to the "Post" button?
Of course, it usually helps to share most of the info in the "About This Mac" window. At least the "Processor", "Memory" and "macOS" info.
Lastly, let's not talk about that three-letter word...