Why gamma 30?

The gamma component is zero by default here, however if you hook up a slider you get 30 as the slider is 30 by default..

You can see how the gamma input maps to conventional gamma ranges here:
http://www.filterforge.com/more/help/...ction.html

Your screen gamma is probably somewhere around 2.2, thus if you want to perform operations in linear space you need to configure the gamma to 1/2.2 = 0.4545.. and then finally before going into the result you reapply 2.2 gamma. However these values are in conventioal gamma ranges, and FF do not provide a mapping formula, only that table above So we need to figure out how to convert conventional gamma ranges to FF input range.. sigh

Sticking to the closest values in that table, the first gamma should be just around -40, and the final gamma should be just around 40 if you want to perform adjustments etc in a more linear space.. (e.g. blurs look much better when applied in linear space)

But I don't know whats going on under the hood in FF, it could very well be that they use internal conversion to linear space and then only for the preview displays and final result convert back to gamma 2.2 ....

Here is an interesting podcast that deals with Adobe's color management. Mostly within the context of After Effects and film but much of the theory applies to PS as well. They touch on your point about blurs looking better in linear. Plus they cover some practical tips for working with blend modes in linear 32bit float space.

Stu Maschwitz has a great blog.