Thank you for your quick reply!

I had not enabled cookie clearing. However, after reading your comment, I added "always allow" exception for bazqux just to be sure.

But I had enabled clearing of offline website data, so maybe that is problem. I disabled it and will find out next time browser crashes.

As for browser itself, I am using latest version of Firefox. Firefox is still notorious for extravagant memory usage. Despite having 16GB of RAM, I suspect that may be reason for crashes - many tabs open, not enough memory.

Still would like presets if not too much trouble and you have time! :)

Besides solving my problem, presets would be useful for quickly switching from one appearance to another. That could happen when user switches between laptop and tablet mode, which I do myself from time to time.

That's quite strange. I haven't seen browser crash in years (maybe you have some bad browser plugin?) and even if browser crashes site data should be left in place (even after hardware reset/power off disk data is safe most of the time).

Have you enabled clearing of cookies and site data on browser exit? Perhaps it's reason of appearance settings not being saved. You should add bazqux.com to exceptions list to keep its cookies and site data after browser restart.

Websites have no access to Windows registry or files on you computer. Perhaps I could add a way to save current theme as a preset.

Until I add presets you could use a workaround. If you hover mouse over "Reader font size" or other labels in Appearance dialog you could see current value in a tooltip. You could write it somewhere to restore settings next time.