I use Firefox on Android and often run into web sites that don't work right because they are dependent on some Chrome or Webkit behaviour, and I need to switch the browser to desktop mode to resolve the problem.
Known issue and I'm yet to see a fix (other than "mbasic" which you have to use for messenger anyway).
Almost everything else seems to be fixed with uBlock Origin and/or Reading Mode.
most web problems are now bugs either by the site developer or in a specific browser.
thats a long way from ie6 problems where either you needed a unique site configuration for every browser or supported ie6 only.