Any Thunderbird experts

[DXP55]

I use Thunderbird for my emails - so far it's fine except for a niggly problem.

If I reply to a email and start to type my reply then stop after a line and half to collect my thoughts then start typing again the font has decreased by half.

I can't seem to find any reason for it - So was wondering if anyone else has had same problem

I am on W7 and use Firefox and Thunderbird.

Thanks

Dave