Sorry about that, I was somewhere else at the time.
Opening other browser tabs does not directly slow upload but can due to the following...
Opening an offline tab can consume computational resources on old slow machines which could result in delays to upload packets (thus slower).
Opening an online tab where your internet is bottleknecking upload will result in slower upload while the page loads due to bandwidth having to be shared (especailly on ADSL where upload and download slow each other). This includes opening tabs on other computers sharing the internet connection.
Opening an online tab where the desternation internet is bottleknecking upload will not make a difference unless the tab is with the desternation site in which case it will slow it down as bandwidth is having to be shared.
When any tab is in a no communication state, they will not affect upload at all.