The solution in my case was a missing key in about:config. It is supposed to be there by default, but it is not. Not even after updating iceweasel to 3.6.4. Anyway the solution is:

Open about:config and search for ui.SpellCheckerUnderlineStyle
If it's there you are fine and you can just check the underline style options. If it's not there - create new integer give it the name ui.SpellCheckerUnderlineStyle and the velues are:
0 = no underlining
1 = dotted line
2 = long dots
3 = single straight line
4 = double underline

The change takes effect immediately
Hope this was helpful