Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

Thanks FredMcD, but I did not even have a personal Firewall activated. Also, this only applies to two or three sites -- sites that ran perfectly well in Firefox just days (or a couple of weeks) ago.

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox from this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Thank you cor-el, but unfortunately your suggestion did not affect the issue. Thank you for trying, though.

Well try this use Safari and use that to compare what your seeing what is there and what isn't there to compare.

Thanks for the thought WestEnd, but when Firefox is displaying nothing but a completely white page, there's not much to compare.

Does Firefox find these website if you use the DNS lookup tool on the about:networking page?

You can try to disable IPv6 (check for other possible causes as well).

• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox from this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Thanks cor-el but I already tried the "forget" solution, to no avail. Also, there's no issue wit Firefox being able to find the sites in question; a quick look at the page source shows that the code is there, but Firefox is simply not displaying the pages. (It's also worth reiterating that the first two pages I mention in my original post are pages that until very recenty displayed with no issues.)

At this point I'm about ready to ditch Firefox altogether. Regardless of what benefits it may have, there's no point in using it if it can't perform basic functions even in safe mode, and no one can figure out why.