Can I refuse auto upgrades due to older computer problems?

Have a Dell computer purchased in 2001. Upgrades to increase speed that makes Firefox faster only slows down my computer because it cannot process info at faster speed. Most times my computer will just freeze up and takes a long time before I can change pages or tabs. Keep getting "Firefox not responding" Can't afford a new computer just yet and this one worked fine on slower systems but auto upgrades are killing me. If I could, I would go back a few upgrades to something slower. Where do I stand. Thanks for any help.