Firefox 136+ broke my startpage configuration, so I figured I might as well write up/copy the fix here since it currently only exists on Reddit.

  1. Open the Firefox installation folder. For me, this was /usr/lib/firefox

  2. Under this folder, open the defaults/pref/ folder. If not present, make those two folders.

  3. Create a file called autoconfig.js inside the defaults/prefs folder, and put the following in it. This lets us run our own Javascript to configure the browser. The comment line at the top is required. Use Unix line endings.

//  
pref("general.config.filename", "autoconfig.cfg");  
pref("general.config.obscure_value", 0);  
pref("general.config.sandbox_enabled", false);

  1. Go back to the Firefox program folder

  2. Create a file named autoconfig.cfg

  3. Put the following in the file, replacing the value of newTabURL with the path to your startpage.

//  

/* set new tab page */
ChromeUtils.defineESModuleGetters(this, {
    AboutNewTab: "resource:///modules/AboutNewTab.sys.mjs",
}); 
var newTabURL = "file:///PATH_TO_YOUR_START_PAGE.html";  
AboutNewTab.newTabURL = newTabURL;
  1. Done :)