How to set default web page size in Firefox?

You can set layout.css.devPixelsPerPx to 1.0 (default is -1) on the about:config page. Adjust its value in 0.1 or 0.05 steps (1.1 or 0.9) until icons or text looks right. Modifying layout.css.devPixelsPerPx affects user interface and web pages (global zoom). You can use an extension to correct the appearance of web pages.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

http://kb.mozillazine.org/about:config

This is one of those things that I've found perpetually frustrating with Firefox. I was using NoSquint for a while, but I found it to be a somewhat inconsistent and gnarly plugin to use. And now it's no longer being maintained and doesn't work with the most recent versions of Firefox.

Until the team at Mozilla do the sensible thing and reinstate this basic accessibility feature, I think the best extension-free solution can be found in a comment on this bug ticket. In case that link ever breaks, I'm reposting the contents below:

1. Open Browser Console (https://developer.mozilla.org/en-US/docs/Tools/Browser_Console#Opening_the_Browser_Console)

2. On the command line (https://developer.mozilla.org/en-US/docs/Tools/Browser_Console#Browser_Console_command_line):

   To Set Global Zoom (put whatever you want instead of 1.33) enter: FullZoom._cps2.setGlobal(FullZoom.name,1.33,gBrowser.selectedBrowser.loadContext);

   To Reset (back to 1) enter: FullZoom._cps2.removeGlobal(FullZoom.name,gBrowser.selectedBrowser.loadContext);

To be clear, the number here represents a decimalised version of zoom percentage. So as with the original question, a zoom level of 170% would be expressed as 1.7.

May be a little bit late, but... I was looking to solve the same problem today and this is how I got here. For whoever this may be useful, I found in about:config the setting zoom.minPercent (Firefox 62). This sets the default zoom to all pages to the given value. It is hackish because you cannot zoom-out more (less) than this value, but I could not find anything better. Regards.