Firefox 37.0.2 mailto link doesn't launch compose window in Live Mail

Question

After Win 7->8.1 upgrade, Email links in Firefox 37.0.2 browser now launch my mail client (Windows Live Mail) but not a compose message window in that app. In Windows 7, Firefox option "Email Link" would launch WLM and open a compose message window pre-populated with my email address as sender and web page link. This message was integrated with my address book for autocomplete suggestions for "To". My default mail settings are all correct as evidenced by the fact that WLM DOES launch from mailto. But it seems to launch just the WLM executable without the -compose option.
Another oddity: When I first reset my defaults for mailto to WLM, mailto DID link to the compose message function of WLM, but has not done so since. No other software installs were done. I've checked and rechecked, turned on and off mail default, with no change. Any advice is much appreciated. I've researched this for hours but can't find solution save a registry change which seems edge to me.