Similar issue: questions/1271019, questions/1252054

This article may help. From memory, it is possible to change the value of that preference. https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Do you use any add-ons related to network connections, proxy, or VPN?

Is your computer managed by an IT department?

If there are issues with the DNS service then maybe Firefox switches from TRR only mode (3) to TRR first mode (2).

Do you use any add-ons related to network connections, proxy, or VPN? Is your computer managed by an IT department?

No, add-ons vary depending on which PC I am using, the common add-ons are cookie auto-delete and ublock.

My computer isn't connected to a domain if that's what you mean if it's managed by an IT department?

If there are issues with the DNS service then maybe Firefox switches from TRR only mode (3) to TRR first mode (2).

I made sure the values of:

network.trr.custom_uri
network.trr.default_provider_uri
network.trr.uri

were all the same and followed some of the instructions in the article provided by TGNorth and it seems to have sorted itself out on all of the browsers apart from one where there was a slight difference in 2 of the fields where I did not append the ID for nextDNS logging, I must have forgotten to change it, the network.trr.mode reverted back to 2 on that browser.

Still keeping an eye on it.