Whenever I move my mouse to each tags, I see this 2605 thing in front of the number of followers. What does this 2605 thing mean? 2605 views or 2605 posts has this tags?
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What you're seeing is a glyph from the Unicode Fallback Font. It should be the BLACK STAR character (★, Unicode: U+2605). The font list for that element is Helvetica Neue, Helvetica, Arial, and sans-serif. It seems odd that none of those fonts are on your machine. So I'm wondering if perhaps you're overriding fonts on your Firefox setup.
When you click this element in succession it begins a toggle cycle: