Basically they have a higher priority than regular tabs, mentioned here [0] in context of unloading and the same applies to execution scheduling.

[0] https://hacks.mozilla.org/2021/10/tab-unloading-in-firefox-9...

Interesting. Does this mean I should tell users to avoid pinning tabs when browsing in the Tor Browser Bundle? (Assuming they have set the slider to "safest" to disallow JS)

With JS disabled only media files could cause a memory leak, so the "safest" option covers that.

Thanks!