Let's base it off a shrieker, since a violet fungus is mobile.
Here goes nothing!
A Shrieker doesn't have a Strength score, so the firethorn wouldn't be able to do any damage when it "lashes out".
A shrieker also doesn't have a Dexterity score, so if we want to keep the AC to the equivalent of the original Firethorn's AC8 it'll need a high natural armour.
How about averaging the stats of the two Fungi?
Shrieker: Str —, Dex —, Con 13, Int —, Wis 2, Cha 1
Violet Fungi: Str 14, Dex 8, Con 16, Int —, Wis 11, Cha 9
Firethorn: Str 7, Dex 4, Con 15, Int —, Wis 6, Cha 5
Hmm, I think I'd rather lean toward the Shrieker - Str 10, Wis 6, Cha 1?
Firethorn
Medium Plant
Hit Dice: 6d8+12 (39 hp)
Initiative: -3
Speed: 0 ft.
Armor Class: 12 (-3 Dex, +5 natural), touch 7, flat-footed 12
Base Attack/Grapple: +4/+4
Attack: Thorns
+6? melee (1d12 plus ignite)
Full Attack: Thorns
+6? melee (1d12 plus ignite)
Space/Reach: 5 ft./0 ft.
Special Attacks: Confusing scent, ignite, thorns
Special Qualities:
Low-light vision or blind?, plant traits,
tremorsense 60 ft.?
Saves: Fort +7, Ref +2, Will +0
Abilities: Str 10, Dex 4, Con 15, Int —, Wis 6, Cha 1
Skills: —
Feats: —
Environment: plains or swamps
Organization: Solitary, patch (2-20) or cluster (20-200)
Challenge Rating: 4?
Treasure: None
Alignment: Always neutral
Advancement: 7-9 HD (Medium) ?
Level Adjustment: —
Confusing Scent (Ex): Firethorns emit a mind-affecting perfume which causes
confusion for 1d6 rounds in any creature that does not succeed at a DC X Fortitude save. The scent affects any creature what comes too close to a firethorn (usually 10 ft, but a strong wind can change the range to 5 ft. upwind and 20 ft. downwind).
Ignite (Ex): A firethorn's thorns burst into flames whenever they strike an opponent, doing an additional 1d20 fire damage (DC X Reflex save for half damage).
Thorns (Ex): A firethorn's thorn attack has a +2 bonus to attack rolls. Any creature or object struck by the thorns will be burnt by the thorns bursting into fire (see Ignite).