Please note the error I encountered when using find files on a system.
I couldn’t find a .url file that is formatted like an .ini file so I had to improvise but I think the answer to your problem is to just pluralize both “text” and “first” in your relevance string:
following texts of firsts "=" of variables whose (it contains "xxx.com") of (find files "*.url" of (descendant folders of folder "c:\users"))
If that doesn’t work – could you post example contents of a .url file configured with variables in it?