Possibly errors in their CSS file https://jigsaw.w3.org/css-validator/validator?uri=https%3A%2F%2Flogin.apigee.com%2Flogin&profile=css3&usermedium=all&warning=1&vextwarning=&lang=en

Firefox new engine is something like looks, reads as CSS then out puts the CSS as pixels. So keep your Video Card Drivers up to date.

Please let us know if this solved your issue or if need further assistance.

CSS errors are quite commom, see https://jigsaw.w3.org/css-validator/validator?uri=www.firefox.com&profile=css3&usermedium=all&warning=1&vextwarning=&lang=en

And DON'T put this on me. The page loads fine in Edge, IE and chrome. Only in firefox it takes 5 to 6 seconds on every machine I tested it. This has nothing to do with my Video Card Drivers.

For the record, with the same Fx57 and W10 as yours, the Sign-in page opens immediately for me.

As an addition to my previous answer I won't be able to solve your problem but there are a few things you might try which could help others.

I have a full range of security and privacy settings and add-ons so there is a fair chance that is not your problem - but to be sure, you could try running Firefox with your anti-virus turned off briefly.

For a broader check also try running Firefox in the diagnostic Safe Mode - hold the Shift Key down as you open Firefox.

You mention "certain web pages" as well as Apigee - do they have anything in common.