User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:79.0) Gecko/20100101 Firefox/79.0

Steps to reproduce:

Click Reply or forward on messages from a some sites with Rich Formatting

Verified emails that produce the issue

• Price drop alerts from camelcamelcamel.com
• Slickdeals Alerts from slickdeals.net
• Windows Insider Program Newsletter
• Riteaid.com newsletter
• Google Analytics Newsletter
• Weekly Glassdoor.com Emails
  Common themes among these sites
• has remote content that Thunderbird blocks by default
• source has CSS styles embedded into many places

Actual results:

My reply texts in compose view can be unreadable at times (white on white, gray on black, etc..)
The reply texts are colored as expected and easy to view once outside compose window.

Expected results:

My reply texts in compose view can be easily discerned.
The reply texts are colored as expected and easy to view once outside compose window.

outside compose window ie. message is sent.

Please attach an example message .eml file using Attach New File button.
Which Thunderbird theme are you using?

Dark theme via Thunderbird settings. .eml to follow

FYI restarted with add-ons disabled did nothing to this issue

Open this file with Thunderbird 78 in dark theme
Hamburger > File > Open > Saved Message

Click forward
Described bug behavior appears

However it seems to not affect reply or reply all for this one.

Here you can see email header table being unreadable.. Same applies to the forward text above the forwarded message.

I have tried again with white theme. Appears to affect both built-in mozilla themes.

We fixed this in bug 1639249