How can I clear individual cookies on firefox? 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.
Firefox for Android stores Cookies  in /data/data/org.mozilla.firefox/files/mozilla/{YOUR_PROFILE}/cookies.sqlite. 

You can't peek into Firefox's data directory without having elevated privileges than what is granted to a normal user, call it superuser access. If you have root access then simply pull that file into PC and open it in an SQLite viewer. Search the Cookies using domain name and purge the ones bothering you. Save the changes and push the file back to its location. When doing that, make sure to delete the files cookies.sqlite-shm and cookies.sqlite-wal in the data directory of Firefox. They are temporary files created when cookies.sqlite  is opened.

For a non-rooted device, you may consider pulling the data directory using adb backup. Extract the backup using ABE. Make the changes, pack the directory into a backup file and restore it in the device. I don't guarantee that restore would work.

There are some addons here which you may try and tell us if they work. They should work regardless of superuser access.