Its not really meaningful, because the firmware could be pretty much any arbitrary OS and it would make zero difference to any end user.
Tannenbaum himself didnt even know about Intel using MINIX in their ME firmware until recently, so that should show you how much relevance it has.
Not at all. MINIX was actually Intel's second choice, they tried first to fit Linux into their new x86 based ME. But the maintainers were uncooperative:
https://www.phoronix.com/scan.php?page=news_item&px=MTY4MzM
Intel then submitted similar patches to the MINIX kernel, which subsequently got accepted.
BSD licensed projects see plenty of contributions, they're just not as popular as Linux because of historical reasons. I and most BSD fans blame the AT&T lawsuit for BSD losing popularity and Linux gaining popularity. That being said, BSD is still quite popular, though somewhat niche.
Companies not upstreaming code will happen regardless of the license. Plenty of companies maintain Linux change sets because they're not obligated to release them, but plenty more upstream their changes when not strictly necessary. It just depends on the value proposition of releasing improvements.