Could you try the steps in this article: How to reset the default user agent on Firefox.

Thank you for that. It was a different but similar field name (no .extra for eg) but after modification of 42's to 57's it has behaved. Again, thanks. Perhaps FF dev team needs to work out why that version info wasn't updating.

Thank you for reporting back.

User agent string preferences are optional user configurable overrides, so they will just stick with whatever you or an add-on sets them to be. In order to have Firefox report its own internal user agent string, you need to clear them.

tsunami_australia said

Thank you for that. It was a different but similar field name (no .extra for eg) but after modification of 42's to 57's it has behaved. Again, thanks. Perhaps FF dev team needs to work out why that version info wasn't updating.

You useragent Mozilla/5.0 (Windows NT 10.0; WOW64; rv:42.0) Gecko/20100101 Firefox/42.0 OWASMIME/4.0500

Firefox 4.0 and later useragents are supposed to not be affected by extensions that would make a useragent stuck at version due to extension adding stuff on end.

Though this OWASMIME does not appear to be extension related but a software?