Firmicutes
Firmicutes is one of the major phyla, or broad family groups, of bacteria in the human gut, and it includes many fiber-fermenting and butyrate-making species. Its value here is the relative share of your sample made up by this group.
Firmicutes and Bacteroidetes together make up the bulk of most people's gut communities, so the balance between them is a commonly discussed big-picture feature. Many well-studied beneficial fermenters, such as Faecalibacterium and Roseburia, sit within this phylum.
A higher Firmicutes share means this group takes up a larger part of your sample relative to the others, which mathematically means other groups take up less. Older ideas linking a high Firmicutes-to-Bacteroidetes ratio to specific outcomes have not held up cleanly, so read this as descriptive rather than good or bad, and not as a diagnosis.
A lower Firmicutes share simply means this group occupies a smaller portion of your sample relative to other phyla, which is best interpreted alongside the specific species within it.
There is no established ideal Firmicutes percentage, and the phylum is too broad for a single target to be meaningful. Because the science here is evolving and the value is relative, focus on the more specific species and diversity signals and interpret with a clinician.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.