Go into the options of whatever is working offline, and uncheck that option. Often it's not really an option that just randomly started messing with you, it can mean that something is actually offline and you need to check why to fix it. A bit like running out of gas and asking how to disable running out of gas. You put in more gas before it runs out, in your case, you see WHY it's working offline.
If you add some details to the post as to what is happening, there may be some more troubleshooting that can be done.
Thank you. The problem is that I cannot open my Windows or Firefox browser because the computer says it is "off line." I tried your suggestion but cannot change the "X" that comes up in Tools. In closing Windows I did come across an ancient unseen download that had stalled without telling me which I closed, I tried to be sure that there are no other programs running hidden, but I still get the "X" in Tools and I still can't get on line.