As I understand, the default access to a Mikrotik router is http://192.168.88.1. But Firefox insists on using https even when in Settings I choose "Don't enable HTTPS-Only Mode" on the Security page. Can't get to the router. Any hints? Thanks.

Switch browser
Or
Enable https access ( www-ssl) ( you can get in using ssh unless that's also disabled)

2 options.

Use winbox instead of webfig

That is practically the only way I access my devices

But yes, so 3 options there.

It's working for me with Firefox. Have you tried adding a Firefox bookmark pointed to http://192.168.88.1 then selecting the bookmark?

Thanks to all of you.
Created the bookmark of http:// but when selected it changed to https:// Will report to Firefox.
Went to WinBox, which had failed yesterday when connecting via MAC address or IP. Using the IP of the router worked this time. The router and the notebook are cabled directly at 100 Mbps.
You gotta LOVE computers or you have to move into a tree

Firefox has strict security settings. There is nothing to report to them, this is by design. You can turn off HTTPS ONLY MODE in Firefox, but they are right, it is dangerous to visit HTTP only sites and enter passwords in them. Don't do that. Use Winbox, or set up proper HTTPS for Webfig.

I gave up on webconfig from firefox.
Using WinBox I reached the router and apparently changed the LAN subnet to 192.168.55.0/24--the notebook wired to eth5 on the router now gets from dhcp the IP 192.168.55.200. BUT, Winbox no longer can manage the router because it says there is no route to host (when addressing it on 192.168.55.1). This seems to be a different problem so I'll start a new thread.
Thanks again for your replies.

It is possible to add it as exception for HTTPS-Only Mode in FF settings and for other web sites will be used (there is no need to globally turn off). If local WebFig IP (or hostname if is accessed by it in url) is added there it will work (for me is working in FF on Mac when using url with http:// scheme in it), set exceptions only if local network is considered as trusted. I also Pi-hole admin accessing over http in FF locally...