Question about Preferred Applications

[Joe Zeff]

My sister uses Linux, and has Thunderbird installed.  However, her 
primary email address is at gmail.  She'd like to be able to click on 
mailto: links and have gmail come up instead of Thunderbird.  AFAICT, 
there's no obvious way to do it.  Does anybody out there know how that 
could be automated?  (My sister isn't technically oriented; something 
that takes several steps every time probably won't work.  However, I can 
either set it up for her or talk her through doing it, as long as the 
end result is easy for her to use.)