Hi Pierre, this is a tough one. The default zoom level is stored in a database with site-specific zoom levels, and there's no preference you can set directly to modify that value.

You definitely can do it with a script you run manually in the Browser Console window, which has an optional global command line. The Options page is probably faster...

/* Default Zoom Level Setter for the Browser Console (2020-08-12)

   NOTE: BEFORE YOU CAN RUN THIS SCRIPT, ONE-TIME SETUP:
   Type or paste about:config into the address bar and press Enter
   Click the button promising to be careful
   In the search box type devt and pause while Firefox filters the list
   If devtools.chrome.enabled is false, double-click it to toggle to true

   Paste this entire script into the command line at the bottom of the Browser Console 
   (Windows/Linux: Ctrl+Shift+j; Mac: Command+Shift+j). Then press Enter to run it. 
   Firefox should flip to an open window and ask what percentage you want.
   
   Ref. https://searchfox.org/mozilla-release/source/browser/components/preferences/main.js#1166
*/

var newpct = window.prompt('New default zoom percentage (e.g., 100, 120, ...)?', '150');
if (newpct){
  let cps2 = Cc["@mozilla.org/content-pref/service;1"].getService(Ci.nsIContentPrefService2);
  let nonPrivateLoadContext = Cu.createLoadContext();
  let win = window.browsingContext.topChromeWindow;
  cps2.setGlobal(win.FullZoom.name, parseFloat((newpct / 100).toFixed(2)), nonPrivateLoadContext);
}

@jscher2000 There is now a "Default zoom" option in Preferences->Zoom->Default zoom . I can't seem to find the about:config option to set it. Any ideas?

@jscher2000 There is now a "Default zoom" option in Preferences->Zoom->Default zoom . I can't seem to find the about:config option to set it. Any ideas?

It is not stored in about:config. It is stored in the content-prefs.sqlite database along with site-specific zoom levels (it is associated with a group with a null name).

(You can use this query in a SQLite viewer to list out the zoom levels in the database: SELECT name, value FROM 'prefs' left outer join 'groups' on prefs.groupID = groups.id WHERE settingID = 1 )

You can consider two use separate profiles each with their own default zoom setting and use Sync to keep both profiles synchronized.

• https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles