There's an extension that lets you do that per element with a right-click.

https://addons.mozilla.org/firefox/addon/zoom-page-we/

That, kind of does it.. but it also resets Every Single page I open to 100%, from the 70% i have default zoom set as in firefox.. There were only 2 options I could see to zoom with that, ALL and TEXT ONLY. I did this somehow the other day, i Thought I was holding down like, shift or something, and scrolling. But the text did Not change size, while Everything else did...

Nope, I tried to give that extension a shot again, Its just the same thing as the normal firefox zoom in terms of function. right clicking only give the open to zoom in and out, to switch type, (text only, or full page) autofit to page, and reset for page or text only...

 And then on pages that have scripts that disable your mouse functions,   it doesnt work at all..

is there nothing for this? or a way to set the max font size the way you can set the minimum?

If you open the extension preferences from the Add-ons and Themes page, the Zoom Levels tab enables you to set the default, text, other, min and max and other levels.

So, the extension, I cant say if it really works or not. it messes up firefox in general for me. when I open almost any page, it will load some, I dont even know how much. the page freezes and is stuck continuosly loading. I cannot scroll or click links, or even stop the page load to try and refresh. firefox usually gives me the banner of "whatever extension is slowing down firefox" everything back to normal when page zoom extension is disabled... ive found workaround myself for some of it at least.. but im just guessing this isnt something possible to be able to do..