Hi Vollbracht, as far as I can tell, Firefox's cache settings don't distinguish different types of files. You could temporarily disable the disk cache and see whether that helps:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box above the list, type or paste cache and pause while the list is filtered

(3) Double-click the browser.cache.disk.enable preference to switch the value from true to false

Does that make it easier? More difficult?

This is a good work around - at least when in a longer development session. Thank you! But the described behavior of Firefox should merely be regarded as a bug.

A solution would be, never to cache anything coming with local files. This should be default behavior any how. A solution would be to reload every style for every reloaded page. This should be default behavior as well as even complex css are a couple of bits only. An other solution would be never to cache different file content separately.

I don't know whether all of these solutions are realistic as styles are to be merged cascading. What can I do to follow up any of them?

Hmm, did that help on yours?

When I search the Disk cache page of about:cache (very slow loading 15000+ entries), there are extremely few file:// entries in there (only one after loading a few files). So I'm not sure what's going on.

(I habitually reload pages bypassing the cache -- Ctrl+Shift+r -- so I haven't noticed sticky style sheets from local files.)

I think this solution is better for solving the cache issue within a single page:

1. Open the developer toolbox (F12).
2. Click the settings button (three dots near the top right).
3. Scroll down to the Advanced settings on the bottom right.
4. Check the option "Disable Cache (when toolbox is open)".
5. Refresh the page.

Also try Shift + Ctrl + R.