Your old data isn't lost, as long as you haven't purged any profile folders, but because of potential file incompatibility, older versions of Firefox won't readily use any profile that has run in a later version. You could migrate some files, but it is worth trying to sort out what is happening since you can't (safely) stay on Firefox 99 forever.

I'm not familiar with what might be going on with your connection. Are there any extra factors, such as VPN software? Does it make any difference to turn off DNS over HTTPS (Firefox DNS-over-HTTPS)?

To migrate files:

(A) Use the about:profiles page to access profile folders. The "Root" directory has the useful files.

(B) See this article regarding which ones are the important ones to copy/paste: Recover important data from an old profile

Thank you for your response, but this seems not to be the problem. Since several years I have installed a home network ( 172...... ) with a NAS and a "Fritz!Box", which connects me via the provider with the internet. As I have no general LAN in my home, I use a Powerline adaptor for the location of this PC, which has a LAN connection to my PC and is plugged into the power socket on the other side, same at the other end, where the data are re-converted from the transmission via powerline into Ethernet LAN. When I install FF 99.x, all is OK an I can access the Fritz!Box and the internet. When I install FF 100.x, FF won't either accept the IP address of the Fritz!Box nor any IP address of the internet. At the same time, I can access the NAS via the explorer, which means that the PC connects as usual via the LAN socket and the powerline adaptors. FF 100.x reacts in a way as if I had pulled out the LAN cable. So, with the same set up, FF 99 works, FF 100 not. In an other room, I have direct Ethernet LAN and my notebook works well with FF 100.x. What I didn't do up to now is transferring the notebook and plug it into the Powerline adaptor and have a look, how FF 100.x works in this set up. I will try it tomorrow.

Thank you, but this answer doesn't explain, why FF 99.x works in this set up and update FF 100.x doesn't work, as I have now changed 2 times between FF 99.x and FF100.x without changing anything of the powerline setup. This powerline doesn't work via WIFI, but its works over the copper wires of the 230V power supply of the home. So I assume that with FF 100.x something in the the connection setup has been changed, that FF 100.x can't "look" beyond the ethernet LAN of the PC to subsequent ( and interface ) equipment and its behavior.

Perhaps this might solve the problem, the router has the ability for Wi-Fi, but first I would have to buy a Wi-Fi interface for my PC. Sure, this workaround might help, but the problem is not with me, but with FF 100.x, so I see the solution first at the responsibility of Mozilla FF. There is another workaround, cheap and immediately available: I'm using FF 99.x until this issue has been solved by the developers/programmers of FF.

I looked at the Fritz site, and it mentions that you connect to the box using either http://fritz.powerline/ or its IP address.

Didi Firefox 100+ give you an error message with that address -- what was the message?

I'm not really clear on why the browser needs to talk to the network device rather than Windows just latching onto it automatically as the default network route for all the apps on the system. ??

I don't remember exactly the wording and I don't want to exercise the update to FF 100.x a third time ( and downgrade again ), but as I remember, the error message was similar to "is not accessible". I tried it with the IP address, and I tried to access the internet via FF with the addresses stored in the bookmarks - "not accessible". It looks like I wrote previously, as if I had disconnected the ethernet cable. I also do not understand, why the MS explorer still connects to the NAS, whereas the FF 100.x refuses to connect to the other side of the powerline system and FF 99.x does.

I don't know. Seems once Windows is connected that should be all you need. Could you submit a ticket with Fritz support? Maybe they can help file a bug with Mozilla if they know what is going on.

Did you check for firewall issues ?

It is possible that your firewall or other security software blocks or restricts Firefox without informing you, possibly after detecting changes (update) to the Firefox application.

See:

• https://support.mozilla.org/en-US/kb/server-not-found-connection-problem
• https://support.mozilla.org/en-US/kb/Firewalls
• https://support.mozilla.org/en-US/kb/fix-problems-connecting-websites-after-updating