Try it on another browser and see if it turns out the same.
Ha! I had the same problem on my windows phone, I suffered for 2 years and eureka, I discovered that it was because I was logged into FR!
Once you log in, the custom settings seem to affect the formatting. I don’t know which setting, so I just stay logged out unless I want to reply to a post.
I use Google Chrome on my LG G3 as I can “pinch” to whatever size font I want. Firefox stops at a size I cannot comfortable read without using my reading glasses which I do not always have around me.