Tab Stash uses Firefox's built-in tab-hiding feature to hide stashed tabs instead of closing them entirely. This allows the tab state to be preserved (vs. closing the tab, which loses all the state). If you accumulate too many hidden tabs, Tab Stash will unload them to make sure your browser doesn't slow down.

If you find this bothersome, you can change Tab Stash's settings so that tabs are entirely closed instead of hidden. There is also a "middle ground" option to unload hidden tabs immediately, so they remain open but don't use any resources.

Hope this helps!