At my previous job, a large payment app serving millions of users, I ran the logs through some analyzers and compared that to GA. There was a noticable difference: Firefox did have a noticable larger amount of users if read from the logs than from GA. If you think about it, it is not that strange either.
Similar: according to GA we had 0 people using Brave, Opera Mini or Icecat, yet the logs did show a (statistically insignificant) handfull of users using that.
So, while there is no technical reason why firefox might be underrepresented, there is a practical and explainable difference in some cases.
Edit: when I say "noticable", I mean that it could be percieved, not that it orders of magnitude or even significan enough to matter for the business or our choices.
Similar: according to GA we had 0 people using Brave, Opera Mini or Icecat, yet the logs did show a (statistically insignificant) handfull of users using that.
So, while there is no technical reason why firefox might be underrepresented, there is a practical and explainable difference in some cases.
Edit: when I say "noticable", I mean that it could be percieved, not that it orders of magnitude or even significan enough to matter for the business or our choices.