Установил второй браузер (Opera 56.0.3051.104) и через некоторое время обнаружил, что вместо Firefox в избранном оказался Opera! (Installed a second browser (Opera 56.0.3051.104) and after a while found that instead of Firefox in favorites was Opera!)

Browsers will usually check if they are the default when they start up, so you might try just launching firefox and use its dialog to set that. If it doesn't check automatically, you should be able to do that through the preferences.

You can also make it more explicit (or force it if the above doesn't work) by going to System Settings -> Applications -> Default Applications -> Web Browser and selecting your preferred browser.