Inspired by this comment, I tested myself and found that Zeds killed exclusively by Ground Fire DoT do NOT award XP to Firebug if it was the only source of damage dealt to the Zed, except for Bloats and Crawlers, since killing them as Firebug always awards XP through the second XP Objective. Weirdly, though, if an Alpha Clot were to die to regular Ground Fire caused by a Flamethrower, but then exploded as a result of Zed Shrapnel, it will award XP despite never dealing direct damage to the Alpha. Obviously, if I were to directly shoot *any* of these Zeds I would be awarded XP for killing them regardless of how much damage I dealt to them before they died, making this inconsistency even more confusing.
Here is a video showcasing the above. First I kill a Bloat and Crawler with exclusively ground fire and, as expected, I'm award XP for killing them. Then I kill an Alpha Clot and am awarded no XP. The next Alpha Clot explodes as a result of Zed Shrapnel and I'm awarded XP despite having dealt no direct damage toward the Alpha, only Ground Fire DoT. I found no difference in results using either the Barbeque or Ground Fire skills.
I'm not sure if this is truly a bug or instead a "lack of a feature" because it's unclear what qualifies as "Firebug Weapon" in the XP Objectives for Firebug. Killing a Bloat/Crawler with ground fire DoT satisfies the "Kill with Firebug Weapons" secondary objective to award XP, which would lead you to believe that Ground Fire would count as damage from a Firebug weapon but doing the same to a Clot isn't sufficient for "Dealing Firebug weapon damage" through the first XP objective and thus you will never get XP for killing Zeds on Firebug through Ground Fire DoT alone, something that is surprisingly common with the HRG Incendiary, Molotovs and the Dragonsbreath all being popular weapons that can apply Ground Fire DoT from afar. If ground fire DoT isn't supposed to award XP without a direct damage source, then why do Shrapnel kills caused through only DoT still award XP?
Here is a video showcasing the above. First I kill a Bloat and Crawler with exclusively ground fire and, as expected, I'm award XP for killing them. Then I kill an Alpha Clot and am awarded no XP. The next Alpha Clot explodes as a result of Zed Shrapnel and I'm awarded XP despite having dealt no direct damage toward the Alpha, only Ground Fire DoT. I found no difference in results using either the Barbeque or Ground Fire skills.
I'm not sure if this is truly a bug or instead a "lack of a feature" because it's unclear what qualifies as "Firebug Weapon" in the XP Objectives for Firebug. Killing a Bloat/Crawler with ground fire DoT satisfies the "Kill with Firebug Weapons" secondary objective to award XP, which would lead you to believe that Ground Fire would count as damage from a Firebug weapon but doing the same to a Clot isn't sufficient for "Dealing Firebug weapon damage" through the first XP objective and thus you will never get XP for killing Zeds on Firebug through Ground Fire DoT alone, something that is surprisingly common with the HRG Incendiary, Molotovs and the Dragonsbreath all being popular weapons that can apply Ground Fire DoT from afar. If ground fire DoT isn't supposed to award XP without a direct damage source, then why do Shrapnel kills caused through only DoT still award XP?