Yaru from stable snap
When I right click in Thunderbird to open menu, I see the horizontal line separator is "very black" :P
Sample screenshot:

Could you check if this also happens with the new thunderbird?
Maybe we are lucky and safe work and this does not occur there.
https://www.omgubuntu.co.uk/2018/08/thunderbird-60-release-features
Just checked it myself, the black line is not there:

Are you okay with closing this then?
The update should hit ubuntu in some weeks/months
Awesome. Feel free to close this issue.
Most helpful comment
Just checked it myself, the black line is not there:
Are you okay with closing this then?
The update should hit ubuntu in some weeks/months