My signature in Thunderbird changes font in replies

I have set in my Thunderbird the signature (html format) addition on every replies. My problem is that when Thunderbird add the signature (bellow my answer and above the citation), it changes its font to match the same email font I'm replying to. How to do I make it show the same font I have fixed for all my emails?

I'm using Thunderbird 45.0 in OS X Yosemite.

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