You can check these prefs and if they are bold then reset them via the right-click context menu

• http://kb.mozillazine.org/ui.key.chromeAccess
• http://kb.mozillazine.org/ui.key.contentAccess

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold(user set).
• Preferences can be reset to the default or changed via the right-click context menu.

Neither of those preferences is bold. Any idea what else might be causing the shortcut change?

An extension can change that behavior.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

It wasn't an extension or add-on. I had to manually set those ui.key... preferences in my user.js file. I'm not really sure why this was happening in the first place but I never had issues with Firefox on the Mac (this was the first time in a long time that I was using Firefox for PC).

To be quite honest, it seems far too complex of a solution (ie: if I wasn't a developer, would I have been able to figure it out? probably not). I wish the about:config would've worked but when I tried making changes there, they never stuck so I was forced to go in and edit the js file.

It shouldn't be necessary to make such changes by using user.js to set prefs.
Either some prefs got corrupted or an extension or theme made those changes happen.

Did you copy any files in the Mac profile folder to the PC (Windows) profile folder?

Nope, this was a clean install of Firefox.

I had this problem. I found that a Roaming profile directory was installed that contained a user.js file in it that changed the shortcut key from CTRL to ALT. I deleted the randomly-named profile folder in c:/Users/[myuser]/AppData/Roaming/Mozilla/Firefox/Profiles and then I pasted in the one from c:/Users/[myuser]/AppData/Local/Mozilla/Firefox/Profiles.

This bad user.js file was making a few other settings, including setting the user agent string to "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.0; hi, Mom) Gecko/20020604"

The user.js file is (or was similar to) the one listed here: https://www-archive.mozilla.org/unix/customizing.html

Thanks, eveostay! That worked. (And for the record, what a crazy user agent string...)