Why is the font still so thin in Firefox? I have installed noto sans cjk on Ubuntu 14.04.4, here is my setting.

/etc/fonts/conf.d/69-language-selector-zh-tw.conf
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>

	<!-- Set fonts selection order for Chinese users -->
	<match target="pattern">
		<test qual="any" name="family">
			<string>serif</string>
		</test>
        <test name="lang">
            <string>zh-tw</string>
        </test>
		<edit name="family" mode="prepend" binding="strong">
			<string>Noto Sans CJK TC Medium</string>
			<string>Noto Sans CJK SC Medium</string>
		        <string>Noto Sans CJK JP Medium</string>
		        <string>Noto Sans CJK KR Medium</string>
			<string>AR PL UMing TW</string>
			<string>AR PL UMing HK</string>
			<string>AR PL New Sung</string>
			<string>HYSong</string>
			<string>WenQuanYi Bitmap Song</string>
			<string>AR PL UKai TW</string>
			<string>AR PL UKai HK</string>
			<string>AR PL ZenKai Uni</string>
			<string>DejaVu Serif</string>
			<string>Bitstream Vera Serif</string>
		</edit>
	</match> 
	<match target="pattern">
		<test qual="any" name="family">
			<string>sans-serif</string>
		</test>
        <test name="lang">
            <string>zh-tw</string>
        </test>
		<edit name="family" mode="prepend" binding="strong">
                        <string>Noto Sans CJK TC Medium</string>
                        <string>Noto Sans CJK SC Medium</string>
                        <string>Noto Sans CJK JP Medium</string>
                        <string>Noto Sans CJK KR Medium</string>
			<string>Droid Sans Fallback</string>
			<string>WenQuanYi Zen Hei</string>
			<string>AR PL UMing TW</string>
			<string>AR PL UMing HK</string>
			<string>AR PL New Sung</string>
			<string>HYSong</string>
			<string>AR PL UKai TW</string>
			<string>AR PL UKai HK</string>
			<string>AR PL ZenKai Uni</string>
			<string>DejaVu Sans</string>
			<string>Bitstream Vera Sans</string>
		</edit>
	</match> 
	<match target="pattern">
		<test qual="any" name="family">
			<string>monospace</string>
		</test>
        <test name="lang">
            <string>zh-tw</string>
        </test>
		<edit name="family" mode="prepend" binding="strong">
                        <string>Noto Sans CJK TC Medium</string>
                        <string>Noto Sans CJK SC Medium</string>
                        <string>Noto Sans CJK JP Medium</string>
                        <string>Noto Sans CJK KR Medium</string>
			<string>Droid Sans Fallback</string>
			<string>WenQuanYi Zen Hei Mono</string>
			<string>AR PL UMing TW</string>
			<string>AR PL UMing HK</string>
			<string>AR PL New Sung</string>
			<string>HYSong</string>
			<string>AR PL UKai TW</string>
			<string>AR PL UKai HK</string>
			<string>AR PL ZenKai Uni</string>
			<string>DejaVu Sans Mono</string>
			<string>Bitstream Vera Sans Mono</string>
		</edit>
	</match>
	<match>
        	<test name="family" compare="contains">
                	<string>Noto Sans CJK</string>
	        </test>
	        <edit name="autohint" mode="assign">
        	        <bool>false</bool>
	        </edit>
        	<edit name="hintstyle" mode="assign">
                	<const>hintfull</const>
	        </edit>
	</match>

</fontconfig>

I also add a file in /etc/fonts/local.conf
<fontconfig>
  <match target="pattern">
    <test qual="any" name="family">
      <string>serif</string>
    </test>
    <edit name="family" mode="prepend" binding="strong">
      <string>Noto Sans CJK TC Medium</string>
      <string>Noto Sans CJK SC Medium</string>
      <string>Noto Sans CJK JP Medium</string>
      <string>Noto Sans CJK KR Medium</string>
    </edit>
  </match> 
  <match target="pattern">
    <test qual="any" name="family">
      <string>sans-serif</string>
    </test>
    <edit name="family" mode="prepend" binding="strong">
      <string>Noto Sans CJK TC Medium</string>
      <string>Noto Sans CJK SC Medium</string>
      <string>Noto Sans CJK JP Medium</string>
      <string>Noto Sans CJK KR Medium</string>
    </edit>
  </match>
  <match target="pattern">
    <test qual="any" name="family">
      <string>monospace</string>
    </test>
    <edit name="family" mode="prepend" binding="strong">
      <string>Noto Sans CJK TC Medium</string>
      <string>Noto Sans CJK SC Medium</string>
      <string>Noto Sans CJK JP Medium</string>
      <string>Noto Sans CJK KR Medium</string>
    </edit>
  </match>
  <match>
        <test name="family" compare="contains">
                <string>Noto Sans CJK</string>
        </test>
        <edit name="autohint" mode="assign">
                <bool>false</bool>
        </edit>
        <edit name="hintstyle" mode="assign">
                <const>hintfull</const>
        </edit>
  </match>
</fontconfig>