You can change this in the settings.

• in Firefox press the menu button on the top right or your phone's menu button
• select settings
• select display
• select text size
• press the small 'A' until the text is at its smallest size
• press ok

You will need to use the reload button for the new text size to take affect on tabs you had open before changing this setting.

So can I not use a larger text size? I had it set at medium so that other sites like Google news were easier to read

If you want forum text to be all the same small size, no.