The x86 architecture has a large number of optional instruction sets. One of these, AES-NI, provides accelerated AES instructions. Another one, AVX, is a collection of general-purpose vector instructions. These were announced at the same time, but it seems like a large number of Intel's low-end CPUs (Celerons & Pentiums) only implement AES-NI.

Firefox's current implementation of the AES-GCM encryption mode uses AVX instructions. The bug report is a submission of a modified implementation that avoids them.