Firefox Public Data Report:
https://data.firefox.com/dashboard/usage-behavior
"Has Add-on" has been between 33-38% for the last two years.
And a very good chunk of those (most I'd say) is only running some form of ad blocking.
This is an old post, but in 2009 about 1/3 of FF users had extensions installed. https://blog.mozilla.org/addons/2009/08/11/how-many-firefox-... Without newer data, you'll have to guess whether the new addon model increased or decreased that fraction.
Do you have the data for this? I know I live in a techie bubble, but it sure doesn't fit with my experience or the experiences of others I know who talk about it.