1. Choose the action "Open Browser" from the "Browser" action group.
   

2. Select "Firefox" in the action parameters under the "Browser" field.
   

3. Specify the descriptive list of browser properties and their values that will be set at startup, in the "Properties" field, in JSON format.

   Template:

   {
       "acceptInsecureCerts": true,
       "browserName": "firefox",
       "timeouts":  {
       "implicit": 0,
       "pageLoad": 300003,
   "script": 30003
      },
         "unhandledPromptBehavior": "dismiss",
         "moz:firefoxOptions": {
         "args": ["-profile", "%USERPROFILE%//AppData//Roaming//Mozilla//Firefox//Profiles//ProfileDir"],
         "prefs": {}
       }
   }

4. Replace %USERPROFILE%//AppData//Roaming//Mozilla//Firefox//Profiles//ProfileDir with the actual path to the profile folder.

Example profile folder:

Example of filling in the fields: