After being prompted a few times by FF and reading a few positive reviews I decided to upgrade. Within 20 minutes I regretted my decision. While I didn’t experience startup issues described here, the following issues caused me to downgrade.

• RoboForm didn’t work right — didn’t look like it was notified about current location so it didn’t show proper login option. RoboForm is essential to my productivity and even if issues were limited to it only I would downgrade as well.
• It looked like there is an issue with keystrokes and mouse clicks — in both search and location box I would type a word and sometimes only half of the word would show up. Same with Page Down/Up
• Clicking on the tab would not always switch to that tab
• Ctrl+R or F5 sometimes would not refresh the page until clicking within the page

Luckily downgrading is easy: download the latest 3.0.x version here

A friend and colleague Bora reported having issues with 3.5 on OSX as well.