> Firefox is may freeze the user agent to a two-digit number like "Firefox/99.0."

Opera did that when version went up from 9 to 10. Too many websites looked at first digit of version rather that doing feature discovery back then.

Useragent string of Opera 12 is "Opera/9.80 (Windows NT 6.1; WOW64) Presto/2.12.388 Version/12.18".