RH8: Bold Text in Paragraph Appears Heavier in Firefox

Noticed recently that bolded text in a paragraph appears differently in Firefox than it does in IE and Chrome. In Firefox the bold text is heavier. We're creating webhelp output using RoboHelp 8 (8.0.2.208). Note that we have recently upgraded to Windows 7 and re-installed RoboHelp. We also upgraded to Firefox 9. We were not seeing this issue before the upgrades. See example below.

RoboHelp HTML:

<p><span style="font-weight: bold;">Open Ports</span>. Select to find hosts
with a particular open TCP or UDP port. Enter the port number in the field
provided. You may include up to 10 ports, separating each port with a
comma.</p>

Appears like this in IE and Chrome (matches WYSIWYG and topic preview from RH):

Appears like this in Firefox (heavier bold font):

The paragraph style as defined in the CSS:

p {
font-size: 10pt;
margin-top: 3pt;
margin-bottom: 7.5pt;
font-family: Arial;
}

Any thoughts on why Firefox looks different? Thanks in advance for your feedback!