Use a clean browser for web wallets and exchanges
This is obvious to anyone who understands web, but a lot of browser extensions can read all the data on all the websites you visit. If you use any Bitcoin web wallets, exchanges, or anything financial in a browser – please for your own good, make sure to do it in a browser that’s completely clean.
You can use an alternate browser (Firefox if your default is Chrome), use an anonymous/incognito browser tab, or setup a unique profile for that … what is important is that the browser you use for financials does not have any extensions.
Extensions do declare their requested permissions … but these change over time and it’s hard to keep track – the safest approach is to make sure no extensions are installed on that browser.
This has been exploited already in the wild by some Bitcoin-specific extensions, but a Bitcoin-stealing module can be added to other unrelated extensions – better safe than sorry.