Many websites offer both, a desktop, and a mobile version. The mobile version usually is a slimmed down version optimized for the smaller display of mobile devices. It may also offer less content and functionality than the desktop version of the site. When visiting a web site with a mobile browser it always defaults to the mobile version of the site, which may not have the desired content or functionality. Hence the option to 'Request desktop site' to gain access to the desired content when using a mobile browser.

Ah... right! Many thanks indeed for your explanation. Truly appreciated.

I can only guess .. that sites with contents for mobile versions are probably somewhat 'slimmer' so they are loaded faster, considering less mobile bandwidth and less 'cpu-power' of mobiles, whereas once Firefox is set to load desktop websites, it will often be much slower then..?

Likely one has simply has to give a try and see how it works. Firefox for Android isn't a real champ on speed, though v60 has improved a little, so enabling this option will possibly diminish the small speed improvement again.

Anyway, that aside, thanks again~!

When your problem is fixed, can you mark the topic as 'Solved' please? Thank you.