Resolve obscure CSS bug

General information for the website: We build online shopping portals
Description of requirements/features: We have run into an obscure CSS bug in an application of ours and are hoping for some help.

If you take a look at the website https://poswebportal.azurewebsites.net and specifically the https://poswebportal.azurewebsites.net/Search/ page.

Using Firefox version 46.0.1 and IE 11.3 I am able to induce a scenario where the main part of the web site moves significantly to the right. I have attached an image demonstrating what it looks like on my screen.

This behavior appears to happen intermittently but its trigger is always scrolling (roughly) half way down the page and then back up. Typically I am able to recreate this bug after closing and opening Firefox a number of times and using CTRL+F5 but once the bug occurs it remains until the page is refreshed.

Note the following.

1. I have had someone previously (who we tried to engage) claim that the bug doesn't exist. It does and I have confirmed it across multiple machines. So if you aren't seeing it, just keep trying. Try reloading the browser, make sure you are using Firefox (as it appears most commonly on Firefox), *try closing and reopening the browser* (this seems to work nearly 100% of the time). Try scrolling up and down numerous times. It's hard to reproduce but its there....

2. It sometimes seems like this might be just associated with pressing refresh too many times on the browser; it's not! Sometimes the bug occurs completely randomly and without doing multiple 'refreshes'.

3. The primary HTML section of the page that appears to be 'moving' is the #content section, however none of the css/html attributes required to make it 'move' (i.e. paddings, margins etc) appear to have changed (as best as I can tell).

4. I am not looking for a complete solution to the problem, we can resolve it ourselves but just need some guidance on what might be happening.

5. I have put the application into DEBUG mode and therefore all the CSS and JavaScript should be un-minimized for easier debugging.

Anyone successful in helping us with this issue would be first considered in a number of further projects we have.

Extra notes: PRIOR TO BIDDING PLEASE ENSURE THAT YOU HAVE BEEN ABLE TO SUCCESSFULLY REPRODUCE THE BUG AND PROVIDE A SCREENSHOT TO CONFIRM THIS. DO NOT BID UNLESS YOU HAVE REPRODUCED THE PROBLEM OR YOU WILL BE DECLINED.