Unable to set default browser

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https://bbs.archlinux.org – Using XFCE I can set the default browser from settings/Preferred Applications to either google chrome or firefox both of which are installed. I set chrome as preferred and can get online no problem. The setting will stay for sometimes days or maybe a few minutes before the system 'forgets' what preference has been set and reverts to no preference set.Using thunderbird to open links in emails always opens Firefox despite the default browser being set to chrome. In fact firefox always stays set as default browser under preferences/general and I can have firefox and chrome open at the s (HowTos)