I believe I've narrowed it down to two possible causes:
1) In your page, you have declared "Arial, Helvetica, sans-serif" as fonts, but then in Bootstrap the fonts are declared as "Helvetica Neue, Helvetica, Arial, sans-serif". As visible from the screenshot, you have different fonts in them, which leads me to believe you have either Helvetica or Helvetica Neue installed.
2) Starting with FF 46, some limited support for -webkit- prefixed styles is available, such as font-size-adjust. In the past, that's been known to cause issues.
Locally I was able to get different results, well, not as different as yours, but still different. I believe that's more of a Bootstrap + FF issue and I am not quite sure how it can be mitigated, if at all.
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