Certain images not loading when using threshold: 0.1 - Firefox

## Expected Behavior
Load image

## Current Behavior
Certain images are not loading. I don't know yet what could be affecting it.


Figure not loading:
```

```

Figure loading:
```

```

## Possible Solution
Use threshold 0.0 (default)

## Context
Loading a list, each element with a "profile" picture. So they have the same structure.
Chrome works OK Version 80.0.3987.149 (Official Build) (64-bit)

Config:

```
const observer = lozad('.lozad', {
rootMargin: '200px 0px',
threshold: 0.1
});
observer.observe();
```

## Tested on:

* Browser Name and version: Firefox Developer Edition, 75.0b5 (64-bit), Chrome works OK Version 80.0.3987.149 (Official Build) (64-bit)
* Operating System and version (desktop or mobile): Ubuntu 20.04 Desktop