Sending, receiving, and organizing email in Outlook on the web for business
An IT tech at my employer sent Microsoft a bug report on this issue. An MS tech contacted me via e-mail for details. After sending them the details, the MS tech replied that they are not going to investigate this problem because it affects Firefox but not Edge. This is contrary to the fact that according to this page Firefox should be supported. So it looks like our only recourse is to use a different browser and gripe.
Thank you everyone for your massive patience.
First, I'd like to address the comment that "[Microsoft is] not going to investigate this problem because it affects Firefox but not Edge." This should not be the case, and I'd like to correct that piece of miscommunication here. I work directly with the teams involved in this issue and we've been trying to narrow down how the issue can be reproduced. There was quite a bit of difficulty reproducing the issue when the diagnosis was attempted (whether it was usage patterns, device specs, email settings, body content, or anything that was hindering the team's ability to reproduce the issue on-demand), and I'm extremely grateful for all the useful information shared in this thread, particularly from people who were able to consistently reproduce the problem and identify the likely root cause.
For the moment, while I beg for more patience, you should be able to work around the issue by using a different browser or by turning off "Text predictions". However, as we can all agree, this is not a long-term solution in the slightest. In fact, it's probably not even a medium-term solution, and I sincerely apologize for the inconvenience and problems it has caused.
I do wish I had found this forum thread earlier, but now that a co-worker pointed me here, I will do my best to see the fixes through. Sorry, and thank you, and sorry once again.