It's a 90s thing.

A 90s thing? More like 10s? If anything it's 13 year olds I see doing that. Well, mentally at least.

Well, I was 13 in the 90s, so that part also applies. :-)

Microsoft had a stranglehold on the computing industry in the 1990s due to its near-monopoly on PC operating systems. Their software was also much lower quality, which led to widespread hatred of the company among computer geeks. Hence the derisive dollar sign.

2010s Microsoft was a shadow of its former self, so I think any usage of the dollar sign then is more of a holdover from the 90s than anything else.

Yep, force of habit.

A string variable named M in QBasic, I imagine.

Microsoft