How do I install a previous version of Firefox?

[Hayli]

Firefox apparently updated to a Beta version. Which the xKit add-on doesn't work with. I need this add-on, so could someone please help me?
@Jazylne: Thank you for your suggestion. I'd prefer to do something a bit less drastic.
@David Mark: How do I do that?
@Sum Tung Wong: I'm very confused on what you are suggesting.

@Mark: I unistalled, and downloaded from the page you hyperlinked. I'm still receiving the same message " Beta versions of Firefox are known to cause problems with XKit. If you still want, you can click here to download XKit 7 manually, but XKit might refuse to boot up or cause problems. Unfortunately, I can not provide support for the version of Firefox you use until it comes out of beta. Make sure other versions of XKit and dashboard themes are disabled before you install XKit."