How do you mean changing? FF by default doesn't change tabs. Sounds more like you installed malware.

stevefawcett2 said

For the last 3 days my tabs are constantly changing, its driving me nuts, please help or I will have to switch to another browser, and I do prefer Firefox

Would you disable your add-on 'Video DownloadHelper 7.3.5' and see if that will make a difference  ? The developer updated his add-on three days ago .....

It was Malware, sorted now

Thanks