Possible meanings, in descending order of likelihood,
are: (1) odious, invidious (since the flames are allegorically
the product of his desires); (2) full of emulation (of Britomart
or her success in passing the flames); (3) malicious, generated
by ill-will (directed against Busirane); (4) actuated by envy (of
Britomart))

4 And bade the stubborn flames to yield him way:
5 But cruel Mulciber would not obey
6 His threatful pride, but did the more augment

threatful > {Full of threats, threatening}

7 His mighty rage, and with imperious sway

sway > power, sweep

8 Him forced (maugre) his fierceness to relent,

maugre > notwithstanding; unfortunately, as luck would have it
relent > abate, lessen

9 And back retire, all scorched and pitifully burnt.

311.27

With huge impatience he inly swelt,
2 More for great sorrow, that he could not pas,
Then for the burning torment, which he felt,
4 That with fell woodnesse he effierced was,
And wilfully him throwing on the gras,
6 Did beat and bounse his head and brest full sore;
The whiles the Championesse now entred has
8 The vtmost rowme, and past the +formest+ dore,
The vtmost rowme, abounding with all precious store.

8 formest > formost _1609_

1 With huge impatience he inly swelt,

impatience > {Inability to endure suffering}
inly swelt > inwardly broiled

2 More for great sorrow, that he could not pass,
3 Than for the burning torment which he felt,
4 That with fell woodness he effierced was,

That > [So that]
fell > terrible, fierce
woodness > fury, madness
effierced > {Rendered fierce: WU}

5 And, wilfully him throwing on the grass,

him > himself

6 Did beat and bounce his head and breast full sore;

bounce > thump
full > exceedingly

7 The whiles the championess now entered has

The whiles > Meanwhile

8 The utmost room, and passed the foremost door,

utmost > outermost

9 The utmost room, abounding with all precious store.

311.28

For round about, the wals yclothed were
2 With goodly arras of great maiesty,
Wouen with gold and silke so close and nere,
4 That the rich metall lurked priuily,
As faining to be hid from enuious eye;
6 Yet here, and there, and euery where vnwares
It shewd it selfe, and shone vnwillingly;
8 +Like+ a discolourd Snake, whose hidden snares
Through the greene gras his long bright burnisht backe declares