OSX 10.6.8 Snow Leopard, Macbook Pro 13", May 2011. When composing in Yahoo Mail (Firefox only) the font size is completely unstable, increasing every time return is pressed. Also, the font I see is smaller than the font indicated by the font box. Doesn't happen in Safari. Setting the font in Firefox preferences doesn't cure the problem and only upsets viewing of all other sites. This problem has been occuring for the past few weeks. The only way to write a homogeneous email is to compose it in the inevitably-increasing font-sizes, then select all and set the font size desired before sending. Any minor alterations at this stage will send the fonts screwy again! It's all very strange and no one else has posted about it anywhere. I find Firefox is better overall than Safari except for this weird flaw and saving web pages as single archive files. What could be causing this problem? Firefox is up-to-date. Could buying the Lion OS help? Thanks.