There is no way to open a link in new tab while using an external mouse. Can't find any way to bring up the menu that a normal long press on the screen would bring up while using a mouse. Chrome recognizes this action but it appears as if Firefox does not. None of my mouse buttons allow me to bring up the menu that gives the option for opening a link in a new tab. Holding in left click does nothing until let go then it acts like a normal left click.
I'd rather not use Chrome but I'm using my phone with my mouse more since it's not practical to carry my laptop with me so as of now it's my only choice.