Unable to clear Browser Data

It's possible the wrong size is reported by the system. And the browser then hangs trying to find it and remove. (The cache sizes fresh out of the box are orders of magnitude smaller.)

So this only happened once, or is it reproducible every time you start from scratch e.g. using a new profile?

On my new laptop. When I first installed Firefox and for sometimes after, the cache size was 54MB+. Now, after a week or so, it is mostly about 7MB and occasionally goes up to 54MB. Clearing the cache at 54MB never works - Firefox hangs and must be aborted manually.

On my old laptop the maximum cache size ever seen was < 4MB. Now, occasionally, the cache size goes up to 7MB, I can't remember if there was ever a 54MB cache. But, when the cache size is large deleting it causes a Firefox hang and a manual abort.

Since Firefox hangs there are no Firefox tools that can be used (?). But, if there are some tools that can be used to help identify the exact cause of failure then I will try to use them before a failure. If there are tools that I can use after a hang using another Firefox browser then I will use them to identify enough information to support identifying the root cause of the failure.

Tell me what I can do to help.

Would you be able to file a bug to help capture any potential specifics if the engineers need some more insight?

The long read is bugzilla.mozilla.org/page.cgi?id=bug-writing.html but in your case if you've reproduced on basically a clean profile / fresh install you can skip ahead the various sanity checks mentioned there (of course you're always welcome to download a separate nightly.mozilla.org to see how that one behaves on the side!) as any followup investigations will be advised on the bug.

You can e.g. open the "Browser Console" from "More Tools" menu, place it somewhere you can see it while clicking around browser settings and watch if there are any interesting errors cropping up after you click the Clear button.