Firefox/Thunderbird Deb10 not displaying any text some web

[k829king]

If I navigate to http://www.tuxmachines.org with firefox (68.8.0ser(64bit) Debian Buster 10 LXDE it displays just the graphic content, all text on the website is not displaying.
The same site on epiphany on same unit renders fine.
Also same site on 32bit LMDE2 firefox 60.3.0ser(32-bit) displays fine.
Also same site on Debian 9 firefox 68.8.0ser(64bit) displays fine.

I extracted the web source for each page on all units and they are identical, i.e. the text is there it is just not displaying. Don't have the issue with most sites.

I've also come across similar problem with the UPS web site https://www.ups.com
If I compare between firefox and epiphany
The landing page will show the map of world on both.
But all text content is missing from the firefox, on epiphany you have the label "select a region", a login hyperlink and a menu structure on right with continents and countries to select from.

Similar issue affects Thunderbird, an email from UPS ends up with no text visible in the email body but is fine on Thunderbird on LMDE2.

I've tried safe mode, clear cache, no difference.

These are the only two websites I have come across (so far) with this issue.

So I'm guessing it is font related, and maybe it could have been a result of adding fonts with winetricks needed to get Adobe Reader dialogs to have content.

Any thoughts on how to resolve or things to try?

[Head_on_a_Stick]

maybe it could have been a result of adding fonts with winetricks needed to get Adobe Reader dialogs to have content

^ What does this mean? What did you do, exactly?

Can we see

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fc-match sans-serif

Both of those sites display fine for me under Debian buster GNOME & firefox-esr 68.8.0esr-1~deb10u1.

For comparison purposes:

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empty@E485:~ $ fc-match sans-serif
arial.ttf: "Arial" "Regular"
empty@E485:~ $

[k829king]

k829king wrote:
maybe it could have been a result of adding fonts with winetricks needed to get Adobe Reader dialogs to have content

^ What does this mean? What did you do, exactly?

urm, it means the only vaguely related things I found googling mentioned fonts, sometime ago, I ran winetricks to install some fonts to resolve an issue with Adobe Reader DC not displaying text in (some?) dialogs.
NB the two URL's I've mentioned I only just recently tried to access on this unit so I do not know if they ever displayed okay (which I assume they did) or at what point they stopped.

re

$ fc-match sans-serif

I get

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$ fc-match sans-serif
DejaVuSans.ttf: "DejaVu Sans" "Book"

NB I get the same result on LMDE2, Deb9 LXDE, Deb10 KDE units/vm's (latter two are pretty much as initially installed from Debian ISO i.e. no wine or wintricks) and all display the URL's okay ....

I will look into ways to selectively remove a font at a time to see if resolves (after I have done a backup)

[k829king]

SOLVED
with font-manager eventually I found a font that was not displaying a preview of the font so I disabled it in font-manager and ran

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fc-cache -f -v

The affected sites now loaded with text though their font size was quite small and I had to use mouse wheel to enlarge font size.

Some how some where since the font is now pre-viewable in font-manager, so I have re-enabled it and run

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fc-cache -f -v

and all back to normal.