This is the code generated by Google calendar.
"<br /><iframe width="400" height="400" frameborder="0" scrolling="no" src="https://www.google.com/calendar/embed?height=400&wkst=1&bgcolor=%23FFFFFF&src=os.ts.team%40gmail.com&color=%23A32929&ctz=Pacific%2FApia" style="border-width: 0pt;"></iframe>"
It's called css...the fault is not in editor, you need to give css for different classes rendered in calender inside iframe for example to theme css files - otherwise for example font size of day tabs is changing when you use different default font sizes in browsers. Width 400px for iframe is too wide for side block and can be too small to render whole calender even in middle topics - if you use large font sizes.
IE and FF have a different way to handle overlaps if content does not fit to given window.
In fact theme css does not seem to mess up google calender so much if it is in side html block and if you have set a little more settings for calender itself.
Calender looks different in editor because editor is using different css than other content of moodle.
See the attached image.
Ah - but there is one good reason for that...
Moodle sets minimum and maximum width for side blocks in index.php with
// Bounds for block widths
// more flexible for theme designers taken from theme config.php
$lmin = (empty($THEME->block_l_min_width)) ? 100 : $THEME->block_l_min_width;
$lmax = (empty($THEME->block_l_max_width)) ? 210 : $THEME->block_l_max_width;
$rmin = (empty($THEME->block_r_min_width)) ? 100 : $THEME->block_r_min_width;
$rmax = (empty($THEME->block_r_max_width)) ? 210 : $THEME->block_r_max_width;
and you can change side block widths in theme config.php - if you want - for example with
$THEME->block_l_min_width = 200;
$THEME->block_l_max_width = 200;
$THEME->block_r_min_width = 400;
$THEME->block_r_max_width = 400;
that sets left blocks to 200px and right blocks to 400px.
Still if images etc are wider than given space IE and FF behave differently (not caused by moodle - see for example http://css-tricks.com/the-css-overflow-property/ how tricky IE can be... )
I have opted to use resources and create a web page that opens in a seperate window. I can control the size of the window and the calendar easily. Perhaps a discussion with the site administrator will allow me to follow up on your advice.