If you're experiencing issues accessing the Linksys RE6500 Setup page using Firefox, you can try a few things to fix the problem.

First, try clearing your browser’s cache and cookies. Sometimes, stored data can interfere with the functioning of a webpage. To do this in Firefox, go to the menu bar and click on "History" and then "Clear Recent History." From there, select "Cache" and "Cookies," and then click on "Clear Now." After that, you may try to remove any blocking ads or blocker of pop-ups you might have set up in your web browser if it fails to work. Sometimes, these extenders can mistakenly block legitimate content on a webpage. To do this, go to the menu bar, click "Add-ons," and disable any ad-blocking or pop-up-blocking extenders. If none of the above solutions work, you can try accessing the setup page using a different browser such as Google Chrome or Microsoft Edge. This can help determine if the issue is specifically with Firefox or the setup page itself.

If you're having trouble accessing the Linksys RE6500 setup page using Firefox, try clearing your browser's cache and cookies, disabling any ad blockers or pop-up blockers, or using a different browser you can contact support to fix this issue.