Theme looks kinda wonky in reader mode on Firefox

[capnfabs]

@jasonaowen reports that footnotes don't show at all on Firefox in reader mode.

I've confirmed this on Firefox 72.0.2. It's not related to the new "Margin Notes" feature; it's happening independently of that.

Additionally, Firefox Reader Mode:

• makes all the 'link to section' icons scale extremely large
• Causes videos to not load.
• Formats image captions the same as body text.

All of these things work fine on Safari Reader Mode (Version 13.0.4 (15608.4.9.1.3)).

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I think my inclination is to call this an issue in Firefox more than an issue in this theme; but it's something I'd consider investigating if a lot of people care about it.