Add "Desktop site" option for Linux mobile

[SethFalco]

Can we get a "Desktop site" button for Linux mobile, similar to what you do on other mobile operating systems like Android?

Reference: How do I change to the desktop view in Firefox for Android? 

Some potential implementation ideas:

• If Firefox can detect if it's on mobile, simply show the menu item in the sandwich menu.
• Otherwise, in the customize toolbar UI, have "Desktop site" as one of the unused items. Users can choose to put this in their toolbar or overflow menu.
• Alternatively, add an option in user preferences to enable either mobile UI, or the "Desktop site" menu item.
• Alternatively, add an option somewhere for distribution maintainers to set when packaging Firefox. For example, postmarketOS create a default config for Linux mobile users.

Reference: postmarketOS/mobile-config-firefox - Config for Firefox package in postmarketOS repos 

Some sites, like Slack, actively prevent users from using the web app if they're on a mobile user-agent. There is no way to bypass it without modifying the user-agent.

A workaround is to install an extension like Header Editor. Users can then configure a rule to modify the user-agent header for the respective domains. However, it'd be better if we could have the experience individuals are used to from other mobile operating systems.

This would greatly benefit users of a PinePhone or Librem device, or other rooted Android devices that can run Linux. (Just to be clear, PinePhone + postmarketOS is my daily driver, so this would help a lot!)

[Jon]

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