I have moved from Ubuntu 14.04 to 18.04. I use Sylpheed mail.
To open the email in HTML, I click on the ‘text/html’ fold-down menu and select ‘Open’, or double-click on the HTML option. This opens the email in Firefox. It worked fine in 14.04, but in 18.04 I get the following message in Firefox:
Access to the file was denied
The file at /home/ubuntu/.sylpheed-2.0/mimetmp/mimetmp.html is not readable.
It may have been removed, moved, or file permissions may be preventing access.
When I select 'Open With', it shows: sensible-editor '%s'
Firefox is set as the default browser and Sylpheed is set as the default mail.
When I copy the address from Firefox (file:///home/ubuntu/.sylpheed-2.0/mimetmp/mimetmp.html) and paste into Chromium, the email displays just fine.
Could there be a permissions issue, because I use the Snap Store version of Firefox now?
Added: The problem appears to rest with Firefox. It turns out that Firefox won't diplay .html files located on an external USB drive either ( i.e. file:///media/ubuntu/USB/index.html)