Hello, sheepcore.
I've been running Firefox 36 on the from Developer Channel for the last few weeks and have had no crashes. Also, I just updated to Firefox 36 on their Release Channel, checked a few things, and it works perfectly.
There was a bug which caused Firefox 34+ to crash after adding several bookmarks to BookMacster directly via BookMacster's Firefox menu items, but that's different than what you're describing here, and also it was fixed a month ago.
Maybe there is a conflict with some other extension or something in your Firefox profile. Please
send me a Trouble Zipper report (in BookMacster, click in the menu:
Help >
Trouble Zipper), and when it asks, it would be good to include your Firefox bookmarks, in whatever profile is crashing if you have more than one Firefox profile.
Trouble Zipper will include a list of all of your other extensions. I hope I can reproduce the problem!