It is more complicated than that:
The Flash issue is basically a long-standing memory leak issue, which has been improved over the years but never fully resolved.
The symptoms depend several parameters, which include how many Flash applets have been going on (ads add up VERY quickly, there can be several on a single page), how many Flash intensive pages have been opened in the current FF session (Flash-based game opening and closing new pages within the game, for instance), size of the physical memory, number of processors, and length of the current FF session itself.
Being aware of those issues, I took the habit to close FF and restart it every 4 or 5 hours at the most, which on my computer is sufficient to prevent the full-freeze FF issue.
That specific issue occurs when the cumulative memory leak reaches a critical point, which depends the physical configuration.
As for Opera, it uses the Chromium version of Flash, which is different from the one for FF (and I do not have experience with it to compare symptoms)
As for the issue you describe, it is the one which requires to clear FF cache, restart FF, and sometimes the computer itself.
It is caused by part of the Flash bugs corrupting FF cache.
All of this has been abundantly discussed on various technical forums, and it is one of the reasons why a number of technical sites are calling for an outright ban of Flash usage.
Even Adobe themselves now, apparently, as I was looking to provide a link: http://www.theverge.com/2016/2/10/10957570/google-bans-flash-display-ads-january-2017