Gerald, I am the developer of the Firefox plug-in at Ancestry.com.
As far as the crashing, maybe you could help me by sending some operating system information, and version of Firefox. Also, for the systems we tested we saw good stability with very little crashing, so I hope we help you with your system.
As to the "can't view until I toggle full screen," I saw that when Firefox 2.0 came out, and I worked on that. I thought we solved it. Maybe if we can get enough people with the same symptom, we could figure out the problem.
Please remember, this is a new product, and as a Firefox user myself, one that I would love to make great. Your constructive input is welcomed.