Welcome to the modern age of browsers. 250MB is not a lot for a browser (on any platform).
If the computer is freezing it suggest two things:
- You're running out of RAM, and
- You have an insufficient swap partition. Swap is disk space where Ubuntu can move things from memory temporarily.
Those two together would cause all sorts of horribleness.
The easy and best solution is to spend $15 on another stick of RAM. Somewhere around 2GB should give you a solid (aka don't really have to worry about RAM) desktop experience.
Other than that, rebooting into a Live CD and adding some swap space is the only real option: https://help.ubuntu.com/community/SwapFaq#How_do_I_add_more_swap.3F But you need to appreciate that swap is a thousand times slower than RAM. Don't rely on it.