http://www.mozilla.com/en-US/firefo
features/
You can install FF2 and FF3 in two separate folders and use both at the same time.
The problem your mom had was probably due to either a missing browser plugin, a temporary glitch in your Internet connection, you were using the BETA version, some other conflict on your computer beside the browser or a poorly designed web page. Most likely the problem will not happen again or is fixable.
I think Google's Chrome will be the wave of the future. Though you might want to give it a few months or a year more development before switching to it.