1) Looks like this only occurs on the web version (due to an oversight), and I've fixed it for the next release. The crash is caused by status effects being applied to units (stepping in puddles or being set on fire would also instantly freeze). Some of the changes in the next build are pretty major, so it might take a while to release.
2) You can stop building by middle-clicking-- however, I haven't tested on Firefox, so it might not be handling input events correctly there.
Thanks for your feedback!