It appears that the extension uses a certain sans-serif font for Hebrew that is very bad at displaying nikud. Some fonts that are good at displaying nikud are: David, Narkisim, SBL Hebrew. Most others, especially sans-serif fonts, are terrible at it.
Can this issue be resolved? There is no problem with displaying Hebrew on any computer since Windows XP, so maybe there is no need for a web font at all. Or if there is, can it be a font that displays properly with nikud?