If it's an ordinary webpage in the pop-up, you could try negative zooming (shrinking the page) by pressing Ctrl+- (Ctrl with hyphen or minus). Once you're done, you can reset the site's zoom level by pressing Ctrl+0 (that's a zero). Does that work?

For keyboard shortcuts, see: Keyboard shortcuts - Perform common Firefox tasks quickly. That's probably not everything, but at least the most commonly used ones.

I don't have a good guess as to why the X button did not close the Bookmarks sidebar.

I have this same problem, but I believe it is a major problem and not just an annoyance as in the original poster.

 It seems my laptop displays the proper width but is lacking the last inch at the bottom of the screen, and the websites I visit a lot will have pop up windows that grey out the original screen and has a blue bar at the top of the pop up but if I try to grab the window and move it up so I can get to the necessary selections buttons at the bottom, then I release the window it either slides back to its original position or it pops back to its original position.
   I believe this is a programming problem with the website but I find it hard to believe a web author would do such a stupid thing like this making the website totally useless for those people who use laptops, tablets, and mobile devices. where the screens do not display the normal size of a typical desktop monitor. Knowing that today, of those people who access the internet, 82% of them will access the internet from a laptop, notebook,, tablet, or other mobile devices.  Any web designer who uses this lock in place pop up window should know they are losing 82% of the potential customers simply because they are not able to access the proper buttons at the bottom of the popup window.
   I have been a computer expert nearly as long as the original poster (I missed the text only stage of computer displays) and I cannot figure out any way of getting around this stupid coding mistake in website design,  so the website is useable. So if FoxFire could give me a way I can get past this coding mistake and let me gain access to the un-movable or re-display to a position popup, I guess I will be like the other 82% of surfers today, and simply go to a website that does not make using their site imposable because they are too stupid to know that today's web-surfing customers use other methods to view the internet and who leave their desktops at home.
   I cannot believe web developers don't check their website coding on several devices before posting the site for internet usage, I know I check everything I write on all the devices I can before I will upload the site to the internet, oh well, my customers will get the surfers and they will simply get ignored because surfers cannot use their sites.

By the way, you can get the pixel height of your screen using Firefox's web console. You can open and close that pane using Ctrl+Shift+k.

If you type or paste these and press Enter, you will get the statistics available to, but perhaps not used by, the page.

• full display height: screen.height
• display height above the Windows Task Bar: screen.availHeight

Many sites use the same few JavaScript libraries, so if one of those fails to pay attention to differences in aspect ratio, it can affect many sites. Are there a few particularly egregious examples that you can provide links for?

I have tried that, I even tried all the screen resolutions on my laptop, but all that the zoom in and zoom out does is makes the inaccessible portion of the popup smaller or larger and changing the screen resolution does nothing as well, it seems like the popup is designed to hide the lower portion of the popup, I cannot imagine why anyone would do that, but the Zoom in and Zoom out was my first try on making this portion of the popup accessible, but it was a nice try anyway.

 I am not sure of the original poster but I have tried everything that changes screen size and I even checked to make sure I had the most current display drivers, I have tried things that even included placing two cursors on the screen, where I used one to grab the blue bar ad move the popup up, but when I put the other cursor on the popup it goes back to its original position and when I take the cursor off the pop up it will go back to the moved position that the other cursor is currently holding the popup at, yet if I move the pop up and release it it will still go back to the original position.
 I went so far as to place the website on my laptop that has a touch screen, I would move the popup with one finger and while still holding it I tried to use the mouse and click on the button but when the cursor goes on top of the pop up, even that my finger is holding it up with the touch screen, it still jumps back to the original position so long as the cursor is on the pop up window.
 I am at a real loss on how to get around this I have tried everything by the book and a lot that isn't in any book.

Hi ProfNeal, does it make any difference if Firefox is in full screen mode (usually toggled with the F11 function key)?

Again, some sample URLs and steps-to-recreate the problem would be great.

Note that lightbox type pop-ups can't be moved or resized.
Their position and size is determined by the JavaScript that creates such a window.