Reported at https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_191#class=mw-datatable_no_longer_working_in_Firefox (archived discussion).
Historically styles applying to mw-datatable were loaded on all pages as part of the legacy stylesheet. They have since moved to the mediawiki.pager.tablePager module.
It appears they are used in wikitext, but this was not documented in the code.
This has never worked on the mobile site.
What should have happened instead?:
If this is a community expectation, we should conditionally load the styles based on whether the element is in the page similar to how the mw-collapsible class works. 
If we need further time to think through whether this should be supported officially, we may want to add this conditional in the interim time to the Monobook and Vector skins.
Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:
- Modern Vector is impacted NOW
- Modern, CologneBlue and MonoBook will be impacted during the 3rd August deploy.
- Legacy Vector will be impacted during the 10th August deploy.
Editors can add the following to their user scripts to workaround this issue while it is open: