It is no wise to use an older version of Firefox, as it has known security holes that have been fixed in the more recent version. What problems are you having in 39? Let us know and we can help you resolve them without downgrading.

I have the website called www.mypoints.com com and they support firefox thru version 38.00 I have 39.00 and their program plugin from their site will not store in the browser because it's not compatible

how do i make it compatible ???

System Requirements: Chrome 30 and 31 on Mac OS X, Chrome 30+ on Windows XP, Vista, 7 and 8. Firefox 25 thru 38.00 on Mac OS X, and XP, Vista and 7.

Sorry that you are having that problem. Going back to a less secure version of Firefox isn't the answer. Unless you are a programmer with knowledge of binary code, you can't fix their plugin and then sign it so that Firefox will use it.

IMO, you should ask that website what their problem is - Firefox 39.0 was released over 5 weeks ago and Firefox 40.0 is slated for release in 5 days - next Tuesday the 11th. Either support Firefox in a timely manner or don't support it - it ain't rocket science; you either support what your users are using or you don't.

And if their programmers need help doing their jobs, they should contact Mozilla and ask for help. Expecting some users to use an older version of Firefox with known exploits isn't a good business model.

According to their website:

https://mypoints.zendesk.com/hc/en-us/articles/201807274-Is-MyPoints-compatible-with-all-browsers-

Though MyPoints works well in a wide variety of browsers, we suggest that you use the latest version of one of the following browsers to ensure the highest quality MyPoints experience: MS Internet Explorer AOL Netscape FireFox Mozilla Opera

I'd suggest installing the toolbar and see if it works.