Is Firefox now blocking good websites?

[STJensen]

Since the last update of Firefox, it seem like I can no longer access
websites that I have been accessing for years. That and friends that
send me links that should be perfectly fine (they are for them) I
cannot open.

For example, I no longer open the following websites:

www.dominic-deegan.com (a webcomic)
www.pawn.se (another webcomic)
www.realclearpolitics.com (a non-partisan news-gathering political
website)
www.city-data.com/forum/las-vegas (a city forum)

If it helps, my computer is running Windows XP and I have AVG Anti-
Virus (free version) and Windows Defender running.

If Firefox is blocking the above, how do I get it to unblock them? If
it isn't blocking the above, any suggestions on how I can find out
what is blocking them and how to get it to unblock them would be
greatly appreciated. Thanks!

Scott