tabs suddenly close on me. I think I may have malware or a hacker on my browser

Hello, First, see if your RAM is near 100%. If so, then that is what is causing the problem. Next, try troubleshoot mode (press shift key while launching Firefox) to see if any extensions are causing anything. If not, go to the next way. Next, in the address bar, search about:crashes and to see if errors are actively happening. lastly, you can try refreshing Firefox by doing this : Click menu button, select help and click "more troubleshooting information" After that, click refresh Firefox. All old data is saved in the "Old Firefox data" folder.