If I left click on an italicised link, when I navigate back all the other italicised links have reverted to plain text.

I don't normally return to the main page via the back button, so I just tried it. In Firefox it doesn't un-italicise the threads, but in Chromium it does. I've no idea why there is a difference. Maybe browsers do behave differently in how they handle the back button. Or maybe my addon just behaves differently.

In both browsers the colour of the thread just visited changes - which (in Firefox at least) seems to do what you want - tells you which threads have been posted to since you last reloaded the page and which of those you've looked at.