Make sure you are not blocking content.

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop also see https://blog.mozilla.org/security/2021/03/23/introducing-smartblock/

https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

Note that Firefox 68 is out of date and is no longer supported with updates.

Verify that you meet the System Requirements (GTK+ and GLib) for the current Firefox release.

• https://www.mozilla.org/en-US/firefox/system-requirements/
• https://www.mozilla.org/en-US/firefox/92.0.1/system-requirements/

Does the text appear if you try to select areas with text with the mouse ?

You can right-click and select "Inspect Element" to open the built-in Inspector with this element selected.

• https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Examine_and_edit_HTML

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Font tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website.

• https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Edit_fonts

Make sure you are not blocking content. ...

Tried in Safe Mode, nothing has changed. Clearing cache & cookies didn't help either.

Note that Firefox 68 is out of date and is no longer supported with updates.

I didn't know that, FF 68 ESR set as default FF browser on Slackware 14.2. and was auto-updated time to time. Couldn't check sys.req. for ver. 68, due to both links redirect to the same place: sys.req. for FF 92.0.1. Selecting any text doesn't show it. On Rules tab the font-family set to "Inter, sans-serif". The Font tab shows: "Inter". Tried to change colour or font-family manually didn't help too.

And since FF 68 ESR is no longer supported I moved to the newest FF and not use built in Slackware feature to update FF. Thank you all.