Pages not showing items in drop down menus etc

In the past couple of weeks I've noticed this happening a lot; particularly in YouTube. Menus don't show items (regardless of appearance color mode) until I refresh the pages, or thumbnail images fail to load. Sometimes the images will load after waiting for several minutes. Drop down menus won't sure their items unless I refresh the page. I tried Microsoft Edge and the problem doesn't exist there. I prefer Firefox, but don't like these annoying glitches. I've also noticed, again on YouTube, if I'm watching a Playlist, the comments under the video are not shown IF I have the playlist open on the side, but if I close the playlist or change the video mode to Cinema Mode, the comments suddenly reappear. Again, this issue doesn't occur if I use the Edge browser. I've ruled out my BitDefender Internet Security App and its VPN. Thanks in advance for any help.