[kilroy]

For the purposes of environmental modifiers, how dense is the smoke produced by smoke grenades/thermal smoke grenades?

I'm assuming it's Heavy Smoke, but all the book says is "The cloud obscures vision, applying visibility modifiers for smoke to relevant tests (p. 174)."

Am I just missing where it's defined, or is it at GM discretion?

[Darzil]

Probably GM discretion.

Personally I'd go with Heavy Smoke in an enclosed space, but possibly Medium or Light in an open space, depending on wind conditions/dispersal.

[Rooks]

Smoke bombs from Hard Targets give -3 medium smoke penalty so usually go with that

[Jack_Spade]

I go with -6 full cover. Otherwise why bother?

[Turkish]

If I were the gm, I'd have rates of degrading levels of cover depending on windspeed. In most circumstances, combat is so fast that the extra book keeping may be unnecessary.