You can, when entering a webmail as default mail client, send pages or link to the webmail, but I haven't had tried it.
For opening a mailto link in a webmail, I don't know if this feature works.
You can try:
Options > Mail & News > Mail Config
A+
Mozilla/5.0 (x11; U; Linux x86_64; fr-FR; rv:38.0) Gecko/20100101 Ubuntu/12.04 K-Meleon/76.0
Web:
http://jujuland.pagesperso-orange.fr/
Mail : alain [dot] aupeix [at] wanadoo [dot] fr
Ubuntu 12.04 - Gramps 3.4.9 - Harbour 3.2.0 - Hwgui 2.20-3 - K-Meleon 76.0 rc