Hello again. I did look at Stylish, but this seems to require a setting for each separate web page

Hello again. I did look at Stylish, but this seems to require a setting for each separate web page i might look at, but i hope to install one-time heading-font-size control which will apply to each and every web page i view, based on the text or content size on that page--or alternately, as a fixed size 15 or 16 font.

i also looked in my FF profiles folder but did not see any .css files to use as a guide. and searching FF support for how to write and install a user .css file was not productive.

i seem to remember early FF versions had a choice for user .css or the webpage .css file in the font choices options page. but i can't find this now. (or perhaps it was Opera that had the user .css choice)

if you could point me to an example i could copy which will control the h1, h2, etc. headings as only a little bit larger than the size of the text/content font on every web page the FF displays, then i would like to install that. and please tell me where to install that .css file.