many tabs (& not all secure sites ask you to allow the tab to reload. I have Tab Mix Plus as an extension & I asked them in a forum if they knew about this event or had one of their options would cause this to happen, & they said effectively "not me"! I've also looked in the Firefox Options & it's not there either! ;-(
It is possible that you try to reload a page that has been requested by sending POST data previously entered in a form.
In such a case Firefox will ask for confirmation to send that POST data.
Other possible causes can be this redirect:
Firefox/Tools > Options/Preferences > Advanced > General : Accessibility : [ ] "Warn me when web sites try to redirect or reload the page"
The setting in "Tools > Options > Advanced > General" is meant as an accessibility feature, as you can see by the label of that section, so that people with disabilities or people who use screen readers do not get confused and is not meant as a safety protection to stop redirecting.