Hi Barney, ta for the feedback.
To say we have had some challenges is an understatement and the whole project is now scheduled to go live in November, touch wood and with a fair breeze 
Your Firefox issue though is baffling, so determined to get to the bottom of it as we can't replicate it. 
Gone through our logs and we can see plenty of visitors landing and navigating the forum using firefox browsers.
According to our Google Analytics, here are the top 50 different Firefox versions that visited the forum over the last 30 days (didn't realise there were that many versions!)

Is your firefox one of those versions? You can find your browser version by selecting the About Firefox.

The only other things we can think of, as an old time forum member, I wonder if you are landing on the forum from an old mylocalforums.com bookmark, from when the forum used to be on it's own site.
I wonder if you are you trying to connect to a https version of the old URL? eg like the image below? As this has no certificate, so will report as not secure.

Or are you landing on this URL, our non secure version of the site?

This should load without any problems, and we haven't got any reported cases of issues, but keen to know if this is the URL you are having problems with.
When the new site goes live, all URLs will become secure, and everything will be redirected to https://www.mylocalservices.co.uk
So firefox should show a secure connection, but for unlogged in users, the Google Adsense units do trigger a tracking protection alert. Is this what you are seeing?
This is an ongoing issue for lots of sites as we slide into an adblocker standards battle.

The only other thing I can think of is if you access the forum from a mobile device, which is: m.mylocalservices.co.uk
This again should currently work with or without https, but will be dropped when the new site is launched as the separate mobile site is getting axed.
So does any of that shed any light Barney?
If not, can you upload a screenshot to this post (or email us at info@mylocalservices.co.uk) so we can see exactly what the issue is.
If anyone else is viewing from a firefox browser and experiencing problems, please let us know as we are struggling to replicate the problem.