For a specific website, I clicked on "Show desktop version" link. Unfortunately the desktop version is so broken I cannot use it even to click on "Mobile version" link. Because the website remembers the setting using cookies, I need to clear that specific cookie to get mobile version of the website back.
When I use "Guest mode" in Firefox, the website works which confirms my hypothesis that cookie causes my problems. Therefore the question: How to delete one specific cookie in firefox on android? I tried installing some plugins, but all I got was "Installation failed" toast message on screen.