Hi Paul, if these devices are HTTP only (port 80 or customized port) and don't support HTTPS (port 443), can you tell whether Firefox is erroneously trying to make a secure connection?

Assuming that is not the issue, are you using purely internal (non-routeable) IP addresses, or do you have it configured so your devices have a public IP address that can be reached from the internet?

Sorry but my computer knowledge is very basic especially where to check for things. I have found how to check for the subnet mask number and it is 255.255.255.0 as normal.

Now here's a strange thing. Going onto my mobile phone on the internet again via the Firefox app when I type in the IP address for both my BT Hub 2 router and my QNAP NAS I can access them with no problem. So, it is definately something to do with my laptop settings because as I said before, no matter which web browser I use noe allow me access to my router or NAS.

If you could point me in the right direction as to where to look and what to check for I would be grateful. Or is it a case of re-setting my laptop which will erase all my apps although not files and starting it as though from new?

Sorry but my computer knowledge is very basic especially where to check for things. I have found how to check for the subnet mask number and it is 255.255.255.0 as normal. Now here's a strange thing. Going onto my mobile phone on the internet again via the Firefox app when I type in the IP address for both my BT Hub 2 router and my QNAP NAS I can access them with no problem. So, it is definately something to do with my laptop settings because as I said before, no matter which web browser I use noe allow me access to my router or NAS. If you could point me in the right direction as to where to look and what to check for I would be grateful. Or is it a case of re-setting my laptop which will erase all my apps although not files and starting it as though from new?

Those "devices" are on the intranet side (local / "internal" private network that can only be accessed by authorized users - typically "wired" by cables and more recently via WiFi wirelessly and Bluetooth) side of the Router which separates your network from the internet - WWW.
Yes, resetting your laptop is most likely the cause.

I all but stopped using my local home network for anything except for 3 computers (and one Smart phone) for being able to connect thru my router to the internet for the last 10 years or so. Never did get the internal device to device connections working properly for file transfers and 'grabbing files' from a home-built NAS-type old, slower Pentium 1 PC that I 'fiddled with' in the mid-00's, and as I retired (3rd time/ 3rd "career") during the Recession of '08 - '11 my home intranet network lost its importance for me. ( '98 - '04 I did "dumpster diving" as a hobby = recycling - LOL) to acquire computers (and other usable stuff that had life left in them) from behind smaller commercial buildings & 'retail' strip malls, that many times still worked after being cleaned. Put in new HDD & OS, and I had a working PC to use or to 'donate' to someone who wanted a basic PC to try using the internet and collected a cache of parts to assemble a "working good one" from the parts of those that were DOA; like starting in 1999 thru maybe as late as 2004).

Really not if much direct help to you, as I am at least like 10 years 'behind the curve' now with networking within Firefox.

Overall, I have fallen behind Mozilla development with Firefox since like Firefox 29 (April 29, 2014) and way further behind as of ~ Firefox 70 (October 22, 2019) with changes that impact "security" as you are having. I just can't provide more insight.
Always been poor with "search" functions to look for relevant information, and this fora I have struggle with more that usual (like 12 years here, now). See if any of these articles might help you.
https://support.mozilla.org/en-US/kb/site-information-panel
https://blog.mozilla.org/security/2019/10/15/improved-security-and-privacy-indicators-in-firefox-70/
https://support.mozilla.org/en-US/kb/firefox-options-preferences-and-settings

Five or 6 hours from now other support contributors with much more knowledge than I ever possessed with computer networking and Firefox advanced knowledge should become available to help you further.

Retired automotive repair shop owner / master technician who retired early and is now 70 yo. Been using PC's since 1984 and learned on MS-DOS 2.0 as a computer owner; been using Firefox since 2002 and have gained great knowledge from being involved with 2 different Mozilla / Firefox support fora since then."

Hi Paul, if none of your browsers can open a local IP address, there does seem to be a system issue, but I'm not sure what it is. Perhaps it would be beneficial to check a Windows 10 forum.