Since I'm also using Firefox, yet the quoted text looks Ok, it's not a Firefox browser problem or a platform problem.
The solution lies in a browser configuration change I made years ago, so you'll forgive me for forgetting about it.
The font size selected by phpBB for quoted text is indeed stupidly tiny, or at least it is in the implementation used for WP - I've run phpBB locally and the defaults there were much more useable. Unfortunately I don't have Developer access and can't therefore adjust the defaults used on WP.
Left to their own devices browsers will try to display text as near as requested by the source page as typeface and size; clearly here that's no use at all and needs to be changed.
In Firefox, look on the "Settings" page for the font configuration and near to that, select the "Advanced" option.
You'll see something similar to this below - probably using different colors and certainly with different font types/sizes.
![Image](https://i.imgur.com/VUfjci5.png)
I'm using an HD screen and most of the default typefaces and sizes are too small for my cheap plywood eyes so the font sizes shown there are just my personal choices.
The kicker is the default next to "Minimum font size" - it'll be "None", meaning the browser will simply use that suggested by the page source. Even if it's too small to be useful.
I suggest as a start point setting it to the same size as used for the "Proportional" font; you can tweak as required for the best effect from there and maybe even tweak the other font details too.
If you have another browser tab open on a page showing quoted text you can make the change and then switch to that tab to see the effect immediately - no browser restarts or page refreshes are required.
I checked this in various browsers and they all have the same configuration available, under different names and through various "Settings" terminology but all basically the same thing, with no minimum font size specified.
In the case of Microsoft's "Edge" browser there's a slider instead of a selectable font size. Typically for MS it's fairly meaningless and selectable as "tiny" to "huge" (or words to that effect) but the font sizes are shown in a different area.
Quote font size tweaked, and resolved, on: Firefox, Edge, Vivaldi, and Google Chrome.
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