= Solution
For $k_\parallel\ne0$ the <Alfvén wave> becomes a <firehose instability> when its squared <angular frequency> is negative:
$$
\boxed{p_{\parallel0}-p_{\perp0}>\frac{B_0^2}{4\pi},\qquad \frac{p_{\perp0}-p_{\parallel0}}{p_{\parallel0}}<-\frac{2}{\beta},\qquad \beta=\frac{8\pi p_{\parallel0}}{B_0^2}.}
$$
Equality is marginal, rather than exponential growth. The <pressure anisotropy> must have negative sign; its absolute magnitude alone is insufficient.
For the intergalactic <plasma>, a negative relative <pressure anisotropy> of $-10^{-2}$ reaches this fluid threshold only for $\beta>200$. In particular, the literal pair $\beta=100$ and relative <pressure anisotropy> $-0.01$ is stable because the threshold is $-0.02$. Since the quoted numbers are approximate and allow larger <plasma beta>, this weakly magnetized <plasma> is susceptible, rather than necessarily unstable at every quoted value. For example, $\beta=300$ and relative <pressure anisotropy> $-0.01$ are unstable. Positive <pressure anisotropy> does not excite this branch.
For the fusion-device <plasma>, $\beta\sim10^{-2}$ requires a negative relative <pressure anisotropy> below roughly $-200$, whereas its magnitude is only about $0.1$. The stabilizing <magnetic tension> dominates, so it is not susceptible to this fluid <firehose instability> under the stated conditions. Indeed, nonnegative perpendicular <pressure> requires the relative <pressure anisotropy> to be no smaller than $-1$; at such a small <plasma beta> even this extreme cannot satisfy the threshold.
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