Why is my old iMac running out of memory with only a few apps open?

Check Activity Monitor and sort the Memory column to see which processes are using the most. Four apps can still use a lot of memory, especially if a browser has many tabs or a page that keeps consuming more over time.

If Firefox is the one growing unusually large, a memory leak is possible, but the number alone won't tell you whether Firefox, a page, or an extension is responsible.

Quit Firefox and reopen it, or force quit it if it won't respond. Restarting the Mac can also clear things out for now. Check memory usage again afterward to see if it stays reasonable or starts climbing.

Update Firefox and install any macOS updates available for your iMac. If the problem comes back, close tabs you don't need, particularly ones playing video, and try disabling extensions to narrow down what's causing the growth.

Saved tabs in a tab manager aren't necessarily active browser tabs, so I wouldn't assume they're the cause without checking.

If Firefox keeps ballooning after a restart and update, try another browser for a while. That can help you tell whether the problem is specific to Firefox. Reinstalling Firefox is another option if the issue persists.