Cross-browser testing : Fonts too large in Firefox but displays fine in Chrome and Opera

Hello I've developed a website using mainly Opera and now have come to testing stage. The site displays mostly fine in Chrome, but in Firefox some of the fonts are too large (or the font style is different), and sometimes spill out of the containers and mess up the presentation. Is there any setting I could add to the CSS to fix the appearance? What's different in Firefox that makes the display of fonts different? Thanks.

Tried removing the font styling CSS but this just alters the font style. I think the font style may be consistent but the sizes definitely are not.

All font is taking more space than it should (as in Opera/Chrome)

! Sometimes the text is overlapping the images, when it should be flowing around it. However this is happening only in some divs and paragraphs.