After a Firefox update I noticed a problem with the browser interface being too large for my taste. I had this problem with Opera but the fix I applied with Opera didn't work for Firefox.
Read: https://davescomputertips.com/is-your-opera-interface-too-big
I found a number of solutions online. Here's the first one that worked.
In Firefox, in the URL or Web Address bar type the following and press enter.
about:config
You'll first get a warning to Proceed with Caution. If you wish to continue click on Accept the Risk and Continue
Now type the following or copy and paste it exactly as show:
ui.textScaleFactor
In the line that appears click on the circle for Number and then click on the Add button (the plus sign +)
In the next line that appears, type the number 100 and then click on the check mark at the end to save it.
The results should be instant but if not just restart the browser.
Other options.
These didn't work for me so provided as FYI.
In the same options config screen type or copy and paste this in the search:
layout.css.devPixelsPerPx
The default is -1. It's recommended to try 0 or 1
Or try this in the same page:
browser.display.os-zoom-behavior
The default is 1 so try 0.
In some case, double-click to edit or change or click on the pencil icon on the end of the line. Don't forget to save if the option is there.
You can use the zoom options on the web-pages but unfortunately it's per page.