Go to security / privacy settings in your browser, clear cache, cookies, passwords, everything you can clear. May sort it for you as one of the cookies may be storing your previous preferences and won't allow itself to be overridden.
Anyway, did you find the drop down menu in upper right hand corner of Firefox android browser? In that area there are three dots in a vertical line, tap them once, select "Request Desktop Site" from the menu that appears.
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