You need to edit several things. Firstly, the fire particles Em4 uses. You can edit them in the Em4 editor to be thicker and stronger, but it will cause a decrease in performance, especially on low to mid level systems, Another possible option is to edit the textures in the Emmiters folder and replace it with a less transparent, but more dense looking smoke texture.
To make fires last much longer, you need to go to Specs/materials.xml and edit the burnTime values for each and every line you find there. The best bet is to take the existing value and add a zero at the end.