I tried one of the addons for Firefox, and it works fine, so that will do for me.

Same here, and I agree that some people may not know that it's possible to rotate to horizontal position. Unless my video is for whatever
reason on tiktok, my videos must be in horizontal position, never vertical.
I just watched a video of a tornado on the ground, in perfect view. The video was completely ruined by the person recording, who felt the
need to constantly zoom into the tornado to capture the debris picked up from it... in combination with vertical positioning.
The fact that the video was only 480p doesn't help either.

Vertical (portrait) video recording drives me insane (short drive)

The world to our eyes is actually widescreen.

These vertical videos show lots of sky, lots of ground/floor, and a fraction of the video subject. I like cars, so these videos usually show the person moving the phone from one side to another to get the entire car in the shot, whereas if they just held the phone correctly....argh...

I hope someday smartphone manufacturers develop a camera system that takes "square" video internally and allows it to be played either way. Upload to YT or the like, and it defaults to uploading the widescreen version.

I might be a genius, at least I ain't got no money.

Oh, and TikTok is owned by (...), and in the USA, it seems only embarrassing videos are uploaded. 

I think square sensors are a great idea for phones IMO, no need to worry about if you want your pictures/video in landscape or portrait at the time of capture. I'm surprised no manufacturer has done that yet since the current trend is to slap a multi camera array on the back of phones too, seems trivial to have one with a square sensor for video. Then couple it with some tweaks in software to keep the RAW square sensor data and allow the user to crop for portrait or landscape for sharing to others.

I find the narrow field of view with the vertical format to be quite annoying too, especially coupled with the person filming excessively panning or zooming in and out in an attempt to capture a wider FOV, which wouldn't even needed to happen if they just filmed in landscape.

Yeah, it seems like those TikTok "trends" seem to encourage users to upload those embarrassing videos. I never understood why it's so popular either, it always just seemed like a clone of the now defunct video platform Vine to me. 

I just don't get what people see in the vertical format. As takemorepills says, our eyes are naturally widescreen, so why pan from side to side to capture all of something instead of just turning the phone 

Oh, sorry to say it Joseph, but my brain rebels when people say 'film' when they really mean 'record' or 'video' - there is no film involved here